This makes the interface more predictable and useful. The caller
understands one or more transactions failed, and can learn what happened
with each transaction. We already have this information, so we might as
well return it.

It doesn't make sense to do this for other PackageValidationResult
values because:
- PCKG_RESULT_UNSET: this means everything succeeded, so the individual
  failures are no longer accurate.
- PCKG_MEMPOOL_ERROR: something went wrong with the mempool logic;
  transaction failures might not be meaningful.
- PCKG_POLICY: this means something was wrong with the package as a
  whole. The caller should use the PackageValidationState to find the
  error, rather than looking at individual MempoolAcceptResults.

ninja all check-all