Show exact error message when authentication falied instead of HTML body

When using pecan.abort() on server side, client side will see response
bodies for HTTP errors, which is not readable for end users.

This patch will let mistral client extract error message from header
returned by mistral serivce, currently, it's only used for
authentication failure.

Closes-Bug: #1502840
Change-Id: I20a7f845676ffe23d93334d171332d717994d937
(cherry picked from commit a5e1e73e83)

mistralclient/api/base.py

@@ -135,7 +135,8 @@ class ResourceManager(object):
 
     def _raise_api_exception(self, resp):
         try:
-            error_data = get_json(resp).get("faultstring")
+            error_data = (resp.headers.get("Server-Error-Message", None) or
+                          get_json(resp).get("faultstring"))
         except ValueError:
             error_data = resp.content
         raise APIException(error_code=resp.status_code,

mistralclient/tests/unit/base.py

@@ -24,6 +24,7 @@ class FakeResponse(object):
     def __init__(self, status_code, content=None):
         self.status_code = status_code
         self.content = content
+        self.headers = {}
 
     def json(self):
         return json.loads(self.content)