The default error message for validators ('Not a valid choice.') is not really helpful.

I suggest better default messages, for example:

• OneOf / ContainsOnly: 'invalid value: {input}; use one of {choices}.'
• Range: 'invalid value: {input}; must be between {min} and {max}.'

The error argument could be a callable as well, for example to restrict the enumeration of valid choices to a certain length.

Thanks for Marshmallow and best regards,
Martin