Custom fields: Render 'required' for tokenizer fields

Summary: When building a tokenizer-based edit control for a custom field (e.g. a datasource type), preserve a field validation error whilst building edit controls.

Test Plan:
  - Create custom datasource field, set it to required
  - Observe that 'Required' does not appear next to control
  - Apply patch
  - Observe 'Required' appears next to control

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D17592

src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldTokenizer.php

@@ -14,6 +14,7 @@ abstract class PhabricatorStandardCustomFieldTokenizer
       ->setName($this->getFieldKey())
       ->setDatasource($this->getDatasource())
       ->setCaption($this->getCaption())
+      ->setError($this->getFieldError())
       ->setValue(nonempty($value, array()));
 
     $limit = $this->getFieldConfigValue('limit');