Add Exception information to failed requests

[StefanCuypers]

I was using the Datasync library and got errors during synch. They were returned in the failed requests of the PullResult, but the status showed 200 and 'OK'. It wasn't clear at all what caused the error. I finished by pulling the source code and debugging and found out that the problem was a Guid field that should have been a Guid? field since it could be null. It would be great if that would be visible in the failed requests.

So in PullOpertationsManager on line 140: result.AddFailedRequest(requestUri, ex.ServiceResponse); -> change to also register the exception itself and not just the ServiceResponse.

[adrianhall]

That's a breaking change, so I wouldn't be able to accept it until the 10.x update.

However, I like the idea - why don't you file an issue on it and I'll see what I can do.