keystoneclient should only attempt to decode JSON in msg body if the response was a success

Since errors could be non-JSON encoded strings, the body in the JSON client should only attempt to be decoded on a 2xx success mode.

error from dashboard logging:

Recoverable error: n/a (HTTP 400)
Creating a new keystoneclient connection to http://proxy:5000/v2.0.
REQ: curl -i http://proxy:5000/v2.0/tenants -X GET -H "User-Agent: python-keystoneclient" -H "X-Auth-Token: 8f45e1bca50d420fbc544380109f1fde"
RESP: {'status': '400', 'content-length': 36, 'content-type': 'text/plain'}
RESP BODY: Unable to find the server at haproxy
Could not decode JSON from body: Unable to find the server at haproxy
Request returned failure status.
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/keystoneclient/client.py", line 121, in request
body = json.loads(body)
File "/usr/lib/python2.7/json/_init_.py", line 326, in loads
return _default_decoder.decode(s)
File "/usr/lib/python2.7/json/decoder.py", line 366, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/lib/python2.7/json/decoder.py", line 384, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded
Could not retrieve tenant list.
Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/horizon/users.py", line 155, in authorized_tenants
authd = api.tenant_list_for_token(self._request, token)
File "/usr/lib/python2.7/dist-packages/horizon/api/keystone.py", line 165, in tenant_list_for_token
return c.tenants.list()
File "/usr/lib/python2.7/dist-packages/keystoneclient/v2_0/tenants.py", line 90, in list
return self._list("/tenants%s" % query, "tenants")
File "/usr/lib/python2.7/dist-packages/keystoneclient/base.py", line 65, in _list
resp, body = self.api.get(url)
File "/usr/lib/python2.7/dist-packages/keystoneclient/client.py", line 166, in get
return self._cs_request(url, 'GET', **kwargs)
File "/usr/lib/python2.7/dist-packages/keystoneclient/client.py", line 150, in _cs_request
**kwargs)
File "/usr/lib/python2.7/dist-packages/keystoneclient/client.py", line 130, in request
raise exceptions.from_response(resp, body)
BadRequest: Unable to communicate with identity service: Unable to find the server at haproxy. (HTTP 400)
novaclient connection created using token "08e95a0d6bb4453392aed4016567602c" and url "http://haproxy:8774/v2/a7e78652440641009d1065c163de6d87"
REQ: curl -i http://haproxy:8774/v2/a7e78652440641009d1065c163de6d87/servers/detail?project_id=a7e78652440641009d1065c163de6d87 -X GET -H "X-Auth-Project-Id: a7e78652440641009d1065c163de6d87" -H "User-Agent: python-novaclient" -H "Accept: application/json" -H "X-Auth-Token: 08e95a0d6bb4453392aed4016567602c"