webadmin: Rephrase messages to fit label operations

Since same errors are used for bonding and for labelling, rephrased
the messages to fit both.

Change-Id: I249b8eebaa1dcec35e4c50c30a8e3f440b952188
Bug-Url: https://bugzilla.redhat.com/1115109
Signed-off-by: Lior Vernia <lvernia@redhat.com>