Delivering error message via header in pecan.abort

This patch correlates with [1] in client side, when authentication
failed, the error message should be delivered to client side.

[1]: https://review.openstack.org/232395

Change-Id: I6a6ced466c05849fd9ff3dcd8377a57c9e9b595f
Closes-Bug: #1502840

mistral/context.py

@@ -207,7 +207,11 @@ class AuthHook(hooks.PecanHook):
             else:
                 msg = 'Authentication required'
 
-            pecan.abort(status_code=401, detail=msg)
+            pecan.abort(
+                status_code=401,
+                detail=msg,
+                headers={'Server-Error-Message': msg}
+            )
 
 
 class ContextHook(hooks.PecanHook):