birdc configure exits with 0 on error

Hey! "birdc configure" (or any command in fact) exits with 0 on error. This is a bit annoying as when using "systemctl reload bird", we get no notification there is an error. Looking at the source code, it seems there is no easy way to hack around that. Commands do not report an error code and messages printed are not tagged as errors. Here is a small workaround for systemd: -- Keep it simple to make it faster. - The Elements of Programming Style (Kernighan & Plauger)