Add info text to `validate_colour()` error message

[mccarthy-m-g]

The info text would provide a hint on how to fix the error. Something like (as separate “i” bullets):

• Is/Are your hexadecimal string/strings of the form “#rrggbb” or “#rrggbbaa”?
• Is/Are your colour name/names included in grDevices::colors()?

It would be nice to make this context aware, so only the relevant hint shows up if invalid colours are specified in a single way, and both show up if there are both invalid hex strings and colour names.