py3: use sysstr() to convert ProgrammingError bytes with no unicode error risk
msg.decode('utf8') may fail if msg isn't an ASCII string, and that's possible
as we sometimes embed a filename in the error message for example.
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: