Downgrade log when attach interface can't find resources

Downgrade the log when a resource (e.g. instance is deleted concurrently)
can't be found, no need to LOG.exception because an exception
is already raised and no need to log again.

Change-Id: I260e89c8deab11bc9707abbbbc7bb9a4f88640e3
Closes-Bug: #1322410
Merge "Downgrade log when attach interface can't find resources"