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/derivation-options.cc

@@ -265,7 +265,8 @@ DerivationOptions::getParsedExportReferencesGraph(const StoreDirConfig & store)
         StorePathSet storePaths;
         for (auto & storePathS : ss) {
             if (!store.isInStore(storePathS))
-                throw BuildError("'exportReferencesGraph' contains a non-store path '%1%'", storePathS);
+                throw BuildError(
+                    BuildResult::InputRejected, "'exportReferencesGraph' contains a non-store path '%1%'", storePathS);
             storePaths.insert(store.toStorePath(storePathS).first);
         }
         res.insert_or_assign(fileName, storePaths);