Use existing non-zero exit code if log failure level is met or exceeded

If the log failure level is met or exceeded (i.e., a log message is logged at a level at or above this level), Antora always exits with an exit code of 1. If an extension has already set the exit code to a non-zero value, that exit code is being overwritten. The non-zero exit code set by an extension should win out. In other words, Antora should only set the exit code to 1 if the value is 0 at exit.