Merge pull request
#1742 from takluyver/customexc-check-once...
Merge pull request
#1742 from takluyver/customexc-check-once
Check for custom_exceptions only once
We already catch custom exceptions in run_code and call the custom handler, but we were checking for them again in showtraceback. This just removes the second check, so custom exception handlers can safely call showtraceback().
