Instead of the deprecated "message" member access,
casting to a string invokes the __str__ method of the exception
that is wired to return the message
Added a test of the failure cases of IpRouteCommand::delete_gateway
because they were missing
Running unit and functional tests locally no longer shows the warning
reported in the bug.
Change-Id: Ia79f526aa973ece1145615d65349f860aa3fd465
Closes-Bug: #1466542