Delivering error message via header in pecan.abort
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
StackStorm/mistral,StackStorm/mistral,openstack/mistral,openstack/mistral