Deletes with nargs should follow a consistent pattern

[Terry Howe]

Deletes of resources with nargs should follow a consistent pattern where

* delete failures do not stop the delete of other resources
* a useful message gets printed for each failure
* $? should be non zero for partial failure

Similar to https://review.openstack.org/#/c/199665/

This should also include an updated to the developer docs which
documents the pattern for commands to follow.

[Richard Theis (rtheis)]

Another consideration is whether or not it should be considered a delete failure if the resource does not exist. The question was raised by Brad Behle here: https://review.openstack.org/#/c/278209/3

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/296891

[OpenStack Infra (hudson-openstack)]

Change abandoned by Steve Martinelli (<email address hidden>) on branch: master
Review: https://review.openstack.org/296891
Reason: i believe this work is already finished

[Steve Martinelli (stevemar)]

Several of these have been implemented without referencing this bug. Let's take them on a case by case basis with new bugs.