Handle errors during glance image updates correctly
Handle errors during glance image updates correctly

Before an UnboundLocalError would occur in case an error would happen
during image updates, because of incorrect referencing of a variable.
Moreover the error was muted by exceptions.handle with ignore=True,
and could result in an incorrect success message.
This change removes references to unused image variable and allows
exceptions to be handled by the function, that calls image_update.

Change-Id: I35ea98a146690565582dd9beea62941f97d32474
Closes-Bug: #1464991
(cherry picked from commit 7eb4e3595c5a30f95d976ea84493453730f3c806)

wangxiangyu/horizon,VaneCloud/horizon,izadorozhna/dashboard_integration_tests,NCI-Cloud/horizon,Daniex/horizon,VaneCloud/horizon,Daniex/horizon,VaneCloud/horizon,NCI-Cloud/horizon,izadorozhna/dashboard_integration_tests,wangxiangyu/horizon,VaneCloud/horizon,NCI-Cloud/horizon,Daniex/horizon,wangxiangyu/horizon,Daniex/horizon,wangxiangyu/horizon,NCI-Cloud/horizon