Better error handling for hooks

[benkeen]

When investigating #464 I found that if you delete a module folder that has an active hook for the {template_hook location="head_bottom"} template hook, you get a blank screen. It's unlikely to happen unless an upgrade went terribly wrong, or someone manually removed a module folder rather than uninstalled it. It's an easy fix: just add a check to confirm the existence of the folder prior to attempting to call it.

This can be reproduced by installing the File Upload, Data Visualization module then just deleting the folders & logging out.

[benkeen]

Line 276 of Hooks.class.php:

// this should include a Modules::isValidModule($module_folder)) check
if (!Modules::checkModuleEnabled($module_folder)) {