Allow returning error from AFURLRequestSerialization requestWithMethod

The requestWithMethod:URLString:parameters: method passes a nil argument for the error when calling requestBySerializingRequest:withParameters:error:. It isn't possible to return an error object from a custom request serializer.

This change adds an overridden version of requestWithMethod:URLString:parameters: that also takes an error parameter, allowing calling code to be notified about errors.