Makes sure compute doesn't crash on failed resume.

If the call to the driver to resume an instance raises an exception,
then nova-compute fails to start properly. This can happen during
recovery, for example, where an instance has been deleted so the
backing files are gone but the database is restored to a state
where it exists.

This patch fixes the issue by catching the exception and setting
the instance to error.

Fixes bug 1100430

Change-Id: Ie31f39fcd43174c29bf7a5cb1b6fa29e4c723f97
Merge "Makes sure compute doesn't crash on failed resume."