Fix 'The "error_page" plugin does not exist' error

As per #2652542: Add .install file abilities for Themes → theme files can't actually support .install files and update hooks!?! We'll need to move this to the base distro and then remove the useless .install file here.

Closing in favor of https://github.com/YCloudYUSA/yusaopeny/pull/96

podarok → closed merge request !70