[ SCALE ] Set custom message to display when `valid_chars` is not matched.

A user might not 100% understand what precisely needs to be entered into a field. Just because a regex says it's wrong, doesn't automatically mean the user would understand.

Being able to set a custom message, means we could, for example say:
"entered is not a valid domain name"
"entered is not a valid API key"
Instead of generic warnings.