shade_check does not raise an error but only prints a warning, produces no output file, and exit status 0

We've noticed that `aclgen --shade_check` does not raise an error (as described in the README) but only prints a warning, produces no output file, and exits with status 0. We expected the command to fail in such a case i.e. actually produce both, a more grave log message e.g. level *error* as well as exit status != 0.