Call to LOG_ERROR should not cause process termination

[stze]

It's a bit counter intuitive to let a log function terminate the process. We should change
the implementation of LOG_ERROR to call warnx() instead of errx(), which does not terminate the process. If a termination of the process is needed in case of an error, we should call abort() after LOG_ERROR.

[mkind]

+1. Another way would be to have a different function called LOG_ERROR_AND_ABORT.