http://qs321.pair.com?node_id=106191

Q&A > input and output > How do I print characters with underline or bold? - Answer contributed by Hofmator

As underlining or bolding are very specific operations depending strongly on the OS, why not use good old simple textual measures to get the readers attention, e.g.

Please supply only *one* argument. Please supply only _one_ argument. Please supply only one(!) argument.
This works everywhere where you can print text :)