ArgumentHandler: Support Custom Validation Errors

[ric-evans]

type-Validation

    Many types of validation exceptions are also handled and returned
    as `400 Bad Request` responses if the `type` parameter is a callable
    that raises an exception.
        > HOWEVER, to include a custom validation message in the
          `400 Bad Request` response, you must raise an
          `argparse.ArgumentTypeError(...)`. Otherwise, the
          `400 Bad Request` response will contain a generic
          message, such as 'argument myarg: invalid type'.