API: More i18n cleanup

API: More i18n cleanup

* Use errorformat for action=login Failed responses in non-BC mode.
* We removed 'messageHtml' from action=rollback's response on error, but
  left it for success. Remove it there too, it's even less useful.
* We changed action=watch's reporting of errors, but left the
  mostly-pointless reporting of "success" UI messages. These should be
  handled on the client side.

Change-Id: Ia6c402a4254fbacf4c2c3f125ce8bf0bcc71e509