Always report an error if cobcd compilation was not successful

I'm always stumbling now and then about running cobcd on an environment that does not match the minimal GnuCOBOL version. This changes at least leads to a reported error during cobcd if the compilation did not work - even if this is only because of the cobcd added options. In no case we should just let this issue be unknown, so additional to the message, this ensures an error to be returned.

I think it would be reasonable to check the output of cobc --version during make in a .PHONY target and return a warning if it looks suspicious - but that's a different thing.