Implements hook_field_update_forbid().
Forbid any field update that changes column definitions if there is any data.
function field_sql_storage_field_update_forbid($field, $prior_field, $has_data) {
if ($has_data && $field['columns'] != $prior_field['columns']) {
throw new FieldUpdateForbiddenException("field_sql_storage cannot change the schema for an existing field with data.");
}
}