The expected and the obtained path are now printed as part of the
error message, making comparing them easier when they're both at hand.
The extra rethrow for the hash-mismatch exception in the bmCheck case
has been removed, allowing the path to be registered as in the
non-check case. This makes having both paths at hand a lot more likely!
The determinism check logic was incorrect for content-addressed paths,
since it only ever tried to compare the path produced, even if this
was not the path expected (in the case of fixed-output derivations) or
the path previously produced (in the case of non-fixed CA
derivations). This made little sense, because that would always be the
same path if it exists! The determinism check is therefore now
bypassed for CA paths. Having a correct determinism check for
non-fixed CA derivations and running the diff hook for fixed-output
derivations would be nice, but feels out of scope and bypassing the
inapplicable logic isn't a regression from the previous behaviour.
Change-Id: I5fc14fb477c8c7d2f5bdedad5591af916f72b128