check_log.py: Improve stack trace pattern matching

This fixes the issue where we got no stack trace printed to check_log output for the issue reported in ATR-20657. The problem was that the "stack trace" pattern was missing from the traceback keyword list. The keyword is now added and instead of duplicating the entries in errorRegex, we now just import traceback into errorRegex.

Another change added as requested in the discussion: patterns which are searched for are only printed if --printpatterns option is specified.