Called by 'validate-object-form-view' during form
deserialization to validate a form view field. Default implementation
ensures that the parsed value satisfies any type requirements declared
on the slot (unless the field has a custom writer, in which case this
condition is waived), and functions declared on the 'satisfies' slot
of the form-view-field.

If the field is validated, the function returns true. Otherwise,
returns nil as the first value and an error message as the second
value. Default implementation obtains the error message by calling
'parser-error-message' on the field's parser.

'slot-name' - the name of the slot being validated.
'object' - object being validated.
'field' - form-view-field being validated.
'view' - form-view being validated.
'parsed-value' - the value parsed from the request.