cee28fbc3 Add error string for CLEANSTACK script violation, preventing an "unknown error" if the CLEANSTACK error condition is set. (Mark Friedenbach)
Pull request description:
This prevents an unhelpful "unknown error" from being printed in test logs if the CLEANSTACK error condition is set.
Backport of Core PR11418
We had applied a similar fix to this previously, but this brings our error message inline with Core's (per feedback from https://github.com/Bitcoin-ABC/bitcoin-abc/issues/393)