Upload rejection fails if email template parameters contain non-ASCII text

https://answers.launchpad.net/launchpad/+question/694768 reports an upload that turned out to fail with OOPS-089a231f7518c189dcbda10e3ed6dce4 while attempting to send a rejection email. That OOPS reads:

  UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 10: ordinal not in range(128)

    Traceback (most recent call last):
  Module lp.archiveuploader.uploadprocessor, line 512, in process
    changes_file, self.processor.log))
  Module lp.archiveuploader.uploadprocessor, line 404, in processChangesFile
    upload.do_reject(notify)
  Module lp.archiveuploader.nascentupload, line 769, in do_reject
    changes_file_object=changes_file_object, logger=self.logger)
  Module lp.soyuz.model.queue, line 952, in notify
    mailer.sendAll()
  Module lp.services.mail.basemailer, line 236, in sendAll
    self.sendOne(email, recipient)
  Module lp.services.mail.basemailer, line 211, in sendOne
    ctrl = self.generateEmail(email, recipient)
  Module lp.soyuz.mail.packageupload, line 615, in generateEmail
    email, recipient, force_no_attachments=force_no_attachments)
  Module lp.services.mail.basemailer, line 114, in generateEmail
    body = self._getBody(email, recipient)
  Module lp.services.mail.basemailer, line 185, in _getBody
    body = template % params
UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 10: ordinal not in range(128)

I've looked at the upload itself, and I think the problem here is that the Maintainer field has no email address, which causes parse_maintainer to return an error message as bytes rather than as text:

        m = re_parse_maintainer.match(maintainer)
        if not m:
            raise ParseMaintError(
                "%s: doesn't parse as a valid %s field."
                % (maintainer.encode("utf-8"), field_name))

I think that making the exception be text rather than bytes should fix this, but of course we'll need to reproduce this in the test suite first to confirm my analysis.