When the magnum-api returns an error code 400-600, SessionClient
faultstring did not return to users. It should return a detailed
error message.
This commit was ported from Ironic:
https://review.openstack.org/#/c/142021/
Change-Id: I8bc15c0ddab10f0e117fb6acb2f3995929fa65e2
Closes-Bug: #1520363