Include keystone message when authentication failed

This was lost in the fix for bug #1526944 but it might be useful.

Change-Id: Ifa8a1b3c2ff2ce6ec79328fb77065eb216e0f538
Related-Bug: #1526944

heatclient/common/http.py

@@ -209,7 +209,8 @@ class HTTPClient(object):
         if not ('X-Auth-Key' in kwargs['headers']) and (
                 resp.status_code == 401 or
                 (resp.status_code == 500 and "(HTTP 401)" in resp.content)):
-            raise exc.HTTPUnauthorized("Authentication failed")
+            raise exc.HTTPUnauthorized(_("Authentication failed: %s")
+                                       % resp.content)
         elif 400 <= resp.status_code < 600:
             raise exc.from_response(resp)
         elif resp.status_code in (301, 302, 305):