Stop double json decoding API error messages
Stop double json decoding API error messages

This patch adds support for the fixed error messages that aren't double
JSON encoded.

Change-Id: Ib39f65c89e3e96efddd9fa3b648145ae3d6159d3
Story: #1662228
Task: #19644

openstack/python-ironicclient,openstack/python-ironicclient