[refactor multisig] move nulldummy check to front

Summary:
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

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3673