[IMPROVED] Adding customizable error page to MPOS

* `.htaccess` is used to re-define the error documents
* redirects to `index.php?page=error&action=<code>`
* does not work for MPOS inside subfolders yet

Addresses #734. If this works we can add this to all the other themes.
[IMPROVE] 404 handling

* [ADDED] Error/404 templates to mmcFE & mobile themes
* [IMPROVED] Make 404 default error with unknown page parameter
Merge pull request #748 from TheSerapher/issue-734

Issue 734