Fixed error messages which printed the same prefix twice

Also updated tests to pass with new error messages
Is there a reason we do this (check if it's a string and call getMessageFromCode) in these two methods but not in loginAnonymously, loginViaToken, or loginViaOAuth?  Maybe we're guaranteed that those will not be strings for some reason, but I don't know.

FWIW, I'd be inclined to rework this so that fb.simplelogin.Errors.format() does this work and calls getMessageFromCode() if we pass it a single string.  Looks like right now it just does "message = '';" in that case.  I'm not sure why it wouldn't just look up the error message.  Thoughts?