core: Change to use generic "name exists" message

Where applicable, commands were changed to use the generic "name already
exists" error message, thus preventing duplication of messages that are
the same except the effected type.

Change-Id: Ibbe8d4c2cc2510bba7aab1f4b760fb77528a118d
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>