Remove unused error code and ensure SCRIPT_ERR_CLEANSTACK has an associated error message

Summary: After a long session of debugging, it turns out CLEANSTACK has no error message...

Test Plan:

make check

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

Reviewed By: freetrader, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D229