Add error code when increasing DoS scores

Summary: This makes the logs clearer.

Test Plan:

make check
../qa/pull-tester/rpc-tests.py

Reviewers: freetrader, sickpig, awemany, kyuupichan, #bitcoin_abc

Reviewed By: freetrader, awemany, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D380