#2040 - Deactivate modules that no longer exist.
- added module::deactivate_missing_modules()
- revised module::deactivate() to change log message if the module is missing
- added call to new function in module::get_obsolete_modules_message()
- added call to new function when loading admin/maintenance menu
Merge pull request #190 from shadlaws/fix_2040

#2040 - Deactivate modules that no longer exist.