Check that ``value`` is valid for unwrapping with ``ComputedField.computed_type``

def validate_unwrap(self, value):
        ''' Check that ``value`` is valid for unwrapping with ``ComputedField.computed_type``'''
        try:
            self.computed_type.validate_unwrap(value)
        except BadValueException as bve:
            self._fail_validation(value, 'Bad value for computed field', cause=bve)