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