selftests: forwarding: Exit with error when missing dependencies

We already return an error when some dependencies (e.g., 'jq') are
missing so lets be consistent and do that for all.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>