Note that this is a behavioural change for the error reason of some
transactions, where there are multiple reasons for failure. We do not
however test for such multi-failure scenarios, anywhere.
For example, a multisig spend with both non-null dummy and fail with
non-null signatures in it previously would give
"mandatory-script-verify-flag-failed (..nullfail reason..)",
but now would give
"mandatory-script-verify-flag-failed (..nulldummy reason..)".
Depends on D3625