[migration/pdb] add option to use pdb.post_mortem if traceback is provided Post mortem is a mode where the pdb shell is opened **where** the exception as occured instead at the breakpoint for set_trace. This is way more useful for debugging for the user because is will have the full context of the error. Closes #17219827

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)