Fwd layer crash when server unavailable
Fwd layer crash when server unavailable

The Forward Layer crashes after giving up on retransmission of request if the server is unavailable. Fixed by using the 5.03 SERVICE_UNAVAILABLE, which seemed the most suitable.

```
2017-04-12 18:51:10,333 - Thread-26  - coapthon.client.coap - DEBUG - Exiting receiver Thread due to request
Exception in thread Thread-7:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/usr/local/lib/python2.7/dist-packages/coapthon/forward_proxy/coap.py", line 286, in receive_datagram
    transaction = self._forwardLayer.receive_request(transaction)
  File "/usr/local/lib/python2.7/dist-packages/coapthon/layers/forwardLayer.py", line 34, in receive_request
    return self._forward_request(transaction, (host, port), path)
  File "/usr/local/lib/python2.7/dist-packages/coapthon/layers/forwardLayer.py", line 104, in _forward_request
    transaction.response.payload = response.payload
AttributeError: 'NoneType' object has no attribute 'payload'
```
Gnomjolnir/CoAPthon,Gnomjolnir/CoAPthon,mcfreis/CoAPthon,Tanganelli/CoAPthon,thejdeep/CoAPthon,Tanganelli/CoAPthon,thejdeep/CoAPthon