Fix unclear error when replacing a nic's ip

The current error message ends up saying something like
'192.168.5.5' != 'network1', which is sort of non-intuitive (yes, of
course they are different). Fix this by explaining better what the
erroneous condition actually is.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Thomas Thrainer <thomasth@google.com>