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/unix/build/linux-derivation-builder.cc

@@ -659,7 +659,7 @@ struct ChrootLinuxDerivationBuilder : ChrootDerivationBuilder, LinuxDerivationBu
             throw SysError("setuid failed");
     }
 
-    std::variant<std::pair<BuildResult::Status, Error>, SingleDrvOutputs> unprepareBuild() override
+    std::variant<BuildError, SingleDrvOutputs> unprepareBuild() override
     {
         sandboxMountNamespace = -1;
         sandboxUserNamespace = -1;