docs: String getNamespaceUriForAliasHandlingUnknown(boolean errorOnUnknown, String alias) {
      String result = getUriForAlias(alias);
      if (result == null) {
        Preconditions.checkArgument(
            !errorOnUnknown, "unrecognized alias: %s", alias.length() == 0 ? "(default)" : alias);
        return "http://unknown/" + alias;
      }
      return result;
    }