Fix "app show" NotFound error message
Fix "app show" NotFound error message

Catch the NotFound when trying to find the Plan, and raise
an appropriate NotFound with the proper resource name.

Change-Id: I117bcb76a95fa938684154ae937dbabf1d955833
Closes-Bug: #1450922

stackforge/python-solumclient,openstack/python-solumclient,stackforge/python-solumclient