Change bad-txn-nonfinal => bad-txns-nonfinal

Summary: It is what is used by legacy clients and what is expected by tests.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py getblocktemplate_proposals.py p2p-fullblocktest.py

Reviewers: freetrader, sickpig, #bitcoin_abc

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D143