Improve error formatting

Changes:

* The divider lines are gone. These were in practice a bit confusing,
  in particular with --show-trace or --keep-going, since then there
  were multiple lines, suggesting a start/end which wasn't the case.

* Instead, multi-line error messages are now indented to align with
  the prefix (e.g. "error: ").

* The 'description' field is gone since we weren't really using it.

* 'hint' is renamed to 'msg' since it really wasn't a hint.

* The error is now printed *before* the location info.

* The 'name' field is no longer printed since most of the time it
  wasn't very useful since it was just the name of the exception (like
  EvalError). Ideally in the future this would be a unique, easily
  googleable error ID (like rustc).

* "trace:" is now just "…". This assumes error contexts start with
  something like "while doing X".

Example before:

  error: --- AssertionError ---------------------------------------------------------------------------------------- nix
  at: (7:7) in file: /home/eelco/Dev/nixpkgs/pkgs/applications/misc/hello/default.nix

       6|
       7|   x = assert false; 1;
        |       ^
       8|

  assertion 'false' failed
  ----------------------------------------------------- show-trace -----------------------------------------------------
  trace: while evaluating the attribute 'x' of the derivation 'hello-2.10'
  at: (192:11) in file: /home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/make-derivation.nix

     191|         // (lib.optionalAttrs (!(attrs ? name) && attrs ? pname && attrs ? version)) {
     192|           name = "${attrs.pname}-${attrs.version}";
        |           ^
     193|         } // (lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform && !dontAddHostSuffix && (attrs ? name || (attrs ? pname && attrs ? version)))) {

Example after:

  error: assertion 'false' failed

         at: (7:7) in file: /home/eelco/Dev/nixpkgs/pkgs/applications/misc/hello/default.nix

              6|
              7|   x = assert false; 1;
               |       ^
              8|

         … while evaluating the attribute 'x' of the derivation 'hello-2.10'

         at: (192:11) in file: /home/eelco/Dev/nixpkgs/pkgs/stdenv/generic/make-derivation.nix

            191|         // (lib.optionalAttrs (!(attrs ? name) && attrs ? pname && attrs ? version)) {
            192|           name = "${attrs.pname}-${attrs.version}";
               |           ^
            193|         } // (lib.optionalAttrs (stdenv.hostPlatform != stdenv.buildPlatform && !dontAddHostSuffix && (attrs ? name || (attrs ? pname && attrs ? version)))) {

src/libstore/build/derivation-goal.cc

@@ -87,8 +87,8 @@ void handleDiffHook(
                 printError(chomp(diffRes.second));
         } catch (Error & error) {
             ErrorInfo ei = error.info();
-            ei.hint = hintfmt("diff hook execution failed: %s",
-                (error.info().hint.has_value() ? error.info().hint->str() : ""));
+            // FIXME: wrap errors.
+            ei.msg = hintfmt("diff hook execution failed: %s", ei.msg.str());
             logError(ei);
         }
     }
@@ -439,12 +439,9 @@ void DerivationGoal::repairClosure()
     /* Check each path (slow!). */
     for (auto & i : outputClosure) {
         if (worker.pathContentsGood(i)) continue;
-        logError({
-            .name = "Corrupt path in closure",
-            .hint = hintfmt(
-                "found corrupted or missing path '%s' in the output closure of '%s'",
-                worker.store.printStorePath(i), worker.store.printStorePath(drvPath))
-        });
+        printError(
+            "found corrupted or missing path '%s' in the output closure of '%s'",
+            worker.store.printStorePath(i), worker.store.printStorePath(drvPath));
         auto drvPath2 = outputsToDrv.find(i);
         if (drvPath2 == outputsToDrv.end())
             addWaitee(upcast_goal(worker.makeSubstitutionGoal(i, Repair)));
@@ -877,9 +874,12 @@ void DerivationGoal::buildDone()
                 statusToString(status));
 
             if (!logger->isVerbose() && !logTail.empty()) {
-                msg += (format("; last %d log lines:") % logTail.size()).str();
-                for (auto & line : logTail)
-                    msg += "\n  " + line;
+                msg += fmt(";\nlast %d log lines:\n", logTail.size());
+                for (auto & line : logTail) {
+                    msg += "> ";
+                    msg += line;
+                    msg += "\n";
+                }
             }
 
             if (diskFull)
@@ -1055,12 +1055,9 @@ HookReply DerivationGoal::tryBuildHook()
 
     } catch (SysError & e) {
         if (e.errNo == EPIPE) {
-            logError({
-                .name = "Build hook died",
-                .hint = hintfmt(
-                    "build hook died unexpectedly: %s",
-                    chomp(drainFD(worker.hook->fromHook.readSide.get())))
-            });
+            printError(
+                "build hook died unexpectedly: %s",
+                chomp(drainFD(worker.hook->fromHook.readSide.get())));
             worker.hook = 0;
             return rpDecline;
         } else
@@ -3068,10 +3065,7 @@ void DerivationGoal::registerOutputs()
         auto rewriteOutput = [&]() {
             /* Apply hash rewriting if necessary. */
             if (!outputRewrites.empty()) {
-                logWarning({
-                    .name = "Rewriting hashes",
-                    .hint = hintfmt("rewriting hashes in '%1%'; cross fingers", actualPath),
-                });
+                warn("rewriting hashes in '%1%'; cross fingers", actualPath);
 
                 /* FIXME: this is in-memory. */
                 StringSink sink;
@@ -3359,10 +3353,7 @@ void DerivationGoal::registerOutputs()
                 if (settings.enforceDeterminism)
                     throw NotDeterministic(hint);
 
-                logError({
-                    .name = "Output determinism error",
-                    .hint = hint
-                });
+                printError(hint);
 
                 curRound = nrRounds; // we know enough, bail out early
             }

src/libstore/build/substitution-goal.cc

@@ -146,11 +146,8 @@ void SubstitutionGoal::tryNext()
         && !sub->isTrusted
         && !info->checkSignatures(worker.store, worker.store.getPublicKeys()))
     {
-        logWarning({
-            .name = "Invalid path signature",
-            .hint = hintfmt("substituter '%s' does not have a valid signature for path '%s'",
-                sub->getUri(), worker.store.printStorePath(storePath))
-        });
+        warn("substituter '%s' does not have a valid signature for path '%s'",
+            sub->getUri(), worker.store.printStorePath(storePath));
         tryNext();
         return;
     }

src/libstore/build/worker.cc

@@ -454,10 +454,7 @@ bool Worker::pathContentsGood(const StorePath & path)
     }
     pathContentsGoodCache.insert_or_assign(path, res);
     if (!res)
-        logError({
-            .name = "Corrupted path",
-            .hint = hintfmt("path '%s' is corrupted or missing!", store.printStorePath(path))
-        });
+        printError("path '%s' is corrupted or missing!", store.printStorePath(path));
     return res;
 }