Set message property in ImageDownloadFailed
Set message property in ImageDownloadFailed

Set the correct property, "message". Looks like "_msg_fmt" may have
been erroneously carried over from some other project (Ironic?)

Change-Id: I2aa56da73660794c6dedcbb8a66e84bcec511a9c
Closes-Bug: #1808443
Related-Bug: #1799221

mahak/cinder,openstack/cinder,mahak/cinder,openstack/cinder