Zaqar wsgi transport doesn't have a default error handler

[Thomas Herve]

When some unexpected error happens, zaqar doesn't return anything to the caller, not even a valid http response. We should add a default error handler and return a 500.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/221744

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/221744
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=c154d6b5b2d16734f70cf24ffbe9f6ea1ea03699
Submitter: Jenkins
Branch: master

commit c154d6b5b2d16734f70cf24ffbe9f6ea1ea03699
Author: Thomas Herve <email address hidden>
Date: Wed Sep 9 15:00:36 2015 +0200

    Add a global error handler for wsgi

    This adds a default error handler for wsgi transport so that unexpected
    exceptions return a responder to the caller, instead of just closing the
    connection.

    Change-Id: I9ad6c06d9922237f66110b3c045fc299ca385bbc
    Closes-Bug: #1493823