Oh, so the crash bug fixed in issue2551093 is actually a bug
not a markdown2 misfeature we needed to trap.

Fixed the issue2551093 test since was failing after the patch.
Geesh that is horrible looking html compared to the other renderers.

Generated new markdown test case to make sure we properly handle
the case where markdown formatting raises an exception.

rev 6279:9ec3a9bc4ea5

Thanks for the fix, it's been pushed to CI now.