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.

Manually tested several of the call sites. Unfortunately, it's a bit

tricky writing unit tests for this right now.
Unit tests pass.