Users working with self-hosted GitLab installs sometimes hit errors

saying the GitLab API version could not be found. While we attempt to

store the reasons this may occur, we never actually output them

anywhere, and when authenticating against the repository we prefix with

a redundant "Unknown error" and stack trace, which doesn't help matters

too much.
This redoes this error reporting to provide a more helpful, informative

error message that lists the causes we stored (hopefully providing some

context to this error), along with some helpful hints on what may have

gone wrong (URL resolution issues or SSL certificate trust issues).

They're no longer shown as unknown errors, and the logs aren't full of

scary stack traces.
The generic error handler for the repository form has also received some

very minor tweaks, saying "Unexpected error" instead of "Uknown error"

and saying that the log file "may" have additional details. It also

removes a logging stack trace, which became redundant with the addition

of a similar logging statement in HostingServiceAuthForm.authorize

when generic SSL cert verification was introduced.

Added new unit tests, which pass.
Manually tested using some bad URLs and saw the more reasonable errors

without all the redundancy.