error: ensure ProgrammingError message is always a str
Since this error is internal-only and a runtime error, let's give it a
treatment that makes it behave identically when repr()d on both Python
2 and Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4545
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors