Normalize authentication errors from SVN.

When failing to authenticate with a repository, the SVN backends will
return an error of "callback_get_login required." This looks like a bug
in Review Board, but is actually just an error message meaning that the
credentials were bad.

We now normalize these errors, converting them into an
authentication-related error. The SSL error handling code already did
this, but now all call sites use it.

Reviewed at https://reviews.reviewboard.org/r/6275/