Clarify what an exception from an action means
Clarify what an exception from an action means

When Python actions raise an exception they may not have failed to run.
This may not even be an issue. For example, the OpenStack action
`swift.head_container` will raise an exception is the container doesn't
exist.

This change lowers the exception to a warning but keeps the exception
traceback in the logs. It also changes the wording in the message. We
didn't fail to run the action, rather the action raise an exception.

Change-Id: If9a6a3b98999acae8b80ad4ddeb9d197a628c280

openstack/mistral,openstack/mistral