Return better error when RESTOperation fails auth

If using RESTOperation to talk to a remote server (not TouchDB), if no credential can be found for an auth challenge
it should call -continueWithoutCredentialForAuthenticationChallenge: instead of cancelAuthenticationChallenge:.
This will ensure that the error seen by the client is an HTTP 401 instead of "NSURLErrorDomain error -1012".
Merge branch 'master' of https://github.com/couchbaselabs/CouchCocoa