Give better feedback when a user enters the wrong duration format

[raddevon]

Frederick experienced this earlier.

I'd love to see us support more input formats (#281), but maybe a stop-gap would be to just change this tip to say "ISO 8601 duration" instead of simply "duration."

[fmoor]

This should explicitly stat that something is wrong and give the requirements for correct input values. The current behavior doesn't make it clear that there is anything wrong.