Validation function takes two argument: id for which it is called
and array of fields' values (usually values for entire formset, as defined
in forms.inc.php).
The function must always return an array with an error (or error array)
assigned to a form element (formset name or field path). Even if there are
no errors, key must be set with an empty value.
Validation functions are assigned in $cfg_db['_validators'] (config.values.php).