Fix the incorrect return messages in keypair show and delete

When we delete or show a keypair, if the keypair doesn't exist,
we'll get "The resource could not be found.(HTTP 404)", this
patch will change it to "ERROR: No keypair with a name or ID of
'keypair_name' exists."

Change-Id: Ifebd8d2213c327f3d3fdd672207170aebbe1bb40
Closes-Bug: #1307338