Plumb problems with p4 repos through the webapi.

Right now, if someone tries to post-review against a p4 server with a broken
setup (wrong auth, wrong P4PORT, etc), they'll get a generic 500 error and a
traceback will be sent to the server admins. We can do better by translating
P4Errors into SCMErrors, and then translating SCMErrors into webapi error codes.

This also adds validation of the perforce repository to the creation form, so
hopefully these errors will be caught sooner.

Newly added unit tests.