Simplify handling of statuses for build errors

Instead of passing them around separately, or doing finicky logic in a
try-catch block to recover them, just make `BuildError` always contain a
status, and make it the thrower's responsibility to set it. This is much
more simple and explicit.

Once that change is done, split the `done` functions of `DerivationGoal`
and `DerivationBuildingGoal` into separate success and failure
functions, which ends up being easier to understand and hardly any
duplication.

Also, change the handling of failures in resolved cases to use
`BuildResult::DependencyFailed` and a new message. This is because the
underlying derivation will also get its message printed --- which is
good, because in general the resolved derivation is not unique. One dyn
drv test had to be updated, but CA (and dyn drv) is experimental, so I
do not mind.

Finally, delete `SubstError` because it is unused.

src/libstore/misc.cc

@@ -322,7 +322,10 @@ StorePaths Store::topoSortPaths(const StorePathSet & paths)
         }},
         {[&](const StorePath & path, const StorePath & parent) {
             return BuildError(
-                "cycle detected in the references of '%s' from '%s'", printStorePath(path), printStorePath(parent));
+                BuildResult::OutputRejected,
+                "cycle detected in the references of '%s' from '%s'",
+                printStorePath(path),
+                printStorePath(parent));
         }});
 }