Cleanup TODO in glance/gateway.py for elasticsearch being unavailable

[Matt Riedemann]

This came up in this change https://review.openstack.org/#/c/171279/4/glance/gateway.py to fix bug 1441239. We are going to merge that change with a debug level log message to avoid a string freeze exception in kilo, but then we need to use this bug to close the TODO when liberty opens up.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/190823
Committed: https://git.openstack.org/cgit/openstack/glance/commit/?id=4a932b0d4b2c8083917ba3fa0321971e55f30389
Submitter: Jenkins
Branch: master

commit 4a932b0d4b2c8083917ba3fa0321971e55f30389
Author: Matt Riedemann <email address hidden>
Date: Thu Jun 11 12:46:56 2015 -0700

    Cleanup TODO in glance/gateway.py for elasticsearch being unavailable

    In commit f53584c2688244f4f5b706aadac1875109a463c5 we added a TODO to be
    more obvious about the error when elasticsearch isn't available if you
    try to start the search service (since elasticsearch is an optional
    dependency). We couldn't add new errors at the time because of
    StringFreeze.

    This resolves the TODO by adding the new SearchNotAvailable exception in
    place of the generic ServiceUnavailable exception.

    Closes-Bug: #1441764

    Change-Id: Ia300d9647f372a677dc536e750dd2ff064601c29