Translate fields and correct private message forms. Fixes #763
Always redirect to referer when messages fail to be created, refs #763

It was redirecting to the referer when there was no `receiver_id` in the
params, but rendering the view index when there was a `receiver_id` but
something else was failing.
Now it always redirects to the referer and sets a more detailed flash message.
Update locales, refs #763