Adapt http response error message parsing
Adapt http response error message parsing

Due I05d2bb60b211d1b18611070217b01e3e418eb79a, change the error message
parsing to get error message from response.

Closes-Bug: #1555985
Change-Id: I4da3aff76fe46d1ac9fd90e06cb4931dc9a8d203

openstack/python-magnumclient