code: 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;
    }
docs: Returns the namespace URI to use for serialization for a given namespace alias, possibly using
  a predictable made-up namespace URI if the alias is not recognized.
  
  <p>Specifically, if the namespace alias is not recognized, the namespace URI returned will be
  {@code "http://unknown/"} plus the alias, unless {@code errorOnUnknown} is {@code true} in
  which case it will throw an {@link IllegalArgumentException}.
  
  @param errorOnUnknown whether to thrown an exception if the namespace alias is not recognized
  @param alias namespace alias
  @return namespace URI, using a predictable made-up namespace URI if the namespace alias is not
  recognized
  @throws IllegalArgumentException if the namespace alias is not recognized and {@code
  errorOnUnkown} is {@code true}