Disallow resize if image not available

If a resize is attempted on an instance that was started from an image
that has since been deleted, a resize will fail. This change makes error
reporting a bit cleaner.

This change is needed since in order to actually properly support
resize/migrate when the image is deleted - it is necessary for nova to keep
a copy of the image metadata and re-use it in case of migration/resize.

Fixes bug: 1160773
Fixes bug: 1177001

Change-Id: Ifaea71f79c97046a4cde094e3a5e676772fcceb4
(cherry picked from commit 65a6264c236a779712694d2379cfe4f9e46e2732)
Merge "Disallow resize if image not available" into stable/grizzly