Return response as error if not valid json object

If the response returned by Mistral do not contain a valid json an
exception is thrown by the client due to invalid json format.
If there's a ValueError exception and it's an error code try to return
the response text instead. (Ex: Authorization Failed )

This may be a temporary fix if the BE is able to return the json
error msg returned by keystone.

Change-Id: I540f3aaa35fb15adc04bf0a33e46af8ac614c814
Closes-Bug: #1316298

mistralclient/api/base.py

@@ -133,8 +133,11 @@ class ResourceManager(object):
         return self.resource_class.resource_name + 's'
 
     def _raise_api_exception(self, resp):
-        error_data = get_json(resp)
-        raise APIException(error_data["faultstring"])
+        try:
+            error_data = get_json(resp).get("faultstring")
+        except ValueError:
+            error_data = resp.content
+        raise APIException(error_data)
 
 
 def get_json(response):