Add proper messages to exceptions

[DVDPT]

I've been awhile struggling with an InvalidOperationException that turn out to be a browser navigation error, please add some kind of message when you throw some exception.

On the FacebookSessionClient class line 158:

            if (result.ResponseStatus == WebAuthenticationStatus.ErrorHttp)
            {
                throw new InvalidOperationException();
            }
            else if (result.ResponseStatus == WebAuthenticationStatus.UserCancel)
            {
                throw new InvalidOperationException();
            }

On windows phone 7 version if any of this exceptions occur the error message is

InvalidOperationException: Operation is not valid due to the current state of the object.

It would be nice to have some better way to distinguish between User cancel and Error, if some kind of error occurs it would be nice to loop a fixed number of times and try to log in.