mail: force SMTPException to bytes before wrapping in error.Abort
Pytype wasn't complaining about this for some reason, but PyCharm was and we
already handle another instance of this exception the same way in this function.
Differential Revision: https://phab.mercurial-scm.org/D10473
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors