Add more detail to router set exception message

When trying to set the external-gateway of a router to a network that is an internal network it would be nice to inform the user that it is not possible because it is not an external network.

The console output shows:
    HttpException: Bad Request

It would be nice if I saw the message in the responses body here:

REQ: curl -g -i -X PUT http://10.1.2.20:9696/v2.0/routers/86290947-8edc-4802-ab0d-5c6296a7cee7 -H "User-Agent: openstacksdk/0.9.12 keystoneauth1/2.16.0 python-requests/2.12.4 CPython/2.7.6" -H "Content-Type: application/json" -H "X-Auth-Token: {SHA1}812c4a71c63336091ea46fdfd4657a468f99f5ea" -d '{"router": {"external_gateway_info": {"network_id": "a98d6817-5190-4440-bc42-a6360c5ec65f"}}}'
http://10.1.2.20:9696 "PUT /v2.0/routers/86290947-8edc-4802-ab0d-5c6296a7cee7 HTTP/1.1" 400 161
RESP: [400] Content-Type: application/json Content-Length: 161 X-Openstack-Request-Id: req-f99c4981-47ec-4874-8d5e-ba06a1ba3012 Date: Fri, 20 Jan 2017 20:46:57 GMT Connection: keep-alive
RESP BODY: {"NeutronError": {"message": "Bad router request: Network a98d6817-5190-4440-bc42-a6360c5ec65f is not an external network.", "type": "BadRequest", "detail": ""}}

specifically the message in the error: ""Bad router request: Network a98d6817-5190-4440-bc42-a6360c5ec65f is not an external network."