revset: factor out composing error message for ParseError to reuse This patch defines the composing function not in "ParseError" class but in "revset" module, because: - "_()" shouldn't be used in "ParseError", to avoid adding "from i18n import _" i18n" to "error" module - generalizing message composition of"ParseError" for all code paths other than revset isn't the purpose of this patch we should also take care of showing "unexpected leading whitespace" for some code paths, to generalize widely.