'Fail the waiting Deferred, if it has not already been fired by
C{fileDescriptorReceived}.  The bytes sent along with a file descriptor
are guaranteed to be delivered to the protocol\'s C{dataReceived} method
only after the file descriptor has been delivered to the protocol\'s
C{fileDescriptorReceived}.'

def dataReceived(self, data):

    if (self.waiting is not None):
        self.waiting.errback(Failure(Exception(('Received    bytes    (%r)    before    descriptor.' % (data,)))))
        self.waiting = None
