code: void error(String key, Object... args) {
          if (apiMode) {
              String msg = log.localize(PrefixKind.JAVAC, key, args);
              throw new PropagatedException(new IllegalStateException(msg));
          }
          warning(key, args);
          log.printLines(PrefixKind.JAVAC, "msg.usage", ownName);
      }
docs: Report a usage error.