```
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.
```
Completes backport of core#26646:
https://github.com/bitcoin/bitcoin/pull/26646/commits/264f9ef17f650035882d24083fb419845942a3ac
Depends on D16402.