Deleting a package group that doesn't exist results in a vague warning:

[jdob@pulp1 ~]$ pulp-admin packagegroup delete --repoid pulp-f16 --id foo
Unable to delete Packagegroup [foo] from repository [pulp-f16]

Deleting a package group category that doesn't exist shows a successful delete:

[jdob@pulp1 ~]$ pulp-admin packagegroup delete_category --repoid pulp-f16 --categoryid foo
Package group category [foo] deleted from repository [pulp-f16]

We should pick one behavior and stick with it, at least within the same section of commands (in this case, package groups).