ebtables-compat: add a bridge-specific exit_error function

ebtables-compat: add a bridge-specific exit_error function

Previous to this patch, error reporting in ebtables-compat was like:

% ebtables-compat xxx
(null) v1.4.21: Bad argument : 'xxx'
Try `(null) -h' or '(null) --help' for more information.

While the original ebtables was:

% ebtables xxx
Bad argument : 'xxx'.

With this patch, we switch to:

% ebtables-compat xxx
Bad argument : 'xxx'.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>