There isn't in the current bits, but there will be with the next drop. There will be two ways to do it:
The first way is to take in the Response message as in input, set whatever you want on it and then set error.
The second way is you will be able to throw a WebFaultException<HttpResponseMessage> passing in whatever you response you want.
Either case will cause the pipeline to stop and immediately return a response. In the current bits, it doesn't look like that scenario is supported.
Out of curiosity what type of processor are you writing?