Instead of that funny business, the fixed output checks are not put in
`checkOutputs`, with the other (newer) output checks, where they also
better belong. The control flow is reworked (with comments!) so that
`checkOutputs` also runs in the `bmCheck` case.
Not only does this preserve existing behavior of `bmCheck`
double-checking fixed output hashes with less tricky code, it also makes
`bmCheck` better by also double-checking the other output checks, rather
than just assuming they pass if the derivation is deterministic.
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.
We currently just use this during the build of a derivation, but there is no
reason we wouldn't want to use it elsewhere, e.g. to check the outputs
of someone else's build after the fact.
Moreover, I like pulling things out of `DerivationBuilder` that are
simple and don't need access to all that state. While
`DerivationBuilder` is unix-only, this refactor also make the code more
portable "for free".
The header is private, at Eelco's request.
