Do not try to uninstall a system module and check module and layout key

[blackcoder87]

Description

• Do not try to uninstall a system module.
• Fix error on uninstall action with invalid module key.
• Check if layoutKey could be valid.
• Add error messages and use correct message for uninstall.
• Remove dead variable.

Fatal error: Uncaught Error: Class '\Modules\Aaaa\Config\Config' not found in application/modules/admin/controllers/admin/Modules.php:425

Stack trace:
#0 application/libraries/Ilch/Page.php(243): Modules\Admin\Controllers\Admin\Modules->uninstallAction()
#1 application/libraries/Ilch/Page.php(137): Ilch\Page->loadController()
#2 index.php(68): Ilch\Page->loadPage()
#3 {main}
    thrown in application/modules/admin/controllers/admin/Modules.php on line 425

PHP Fatal error:  Uncaught Error: Class "\Layouts\Layouts\Config\Config" not found in application/modules/admin/controllers/admin/Layouts.php:330
Stack trace:
#0 application/libraries/Ilch/Page.php(243): Modules\Admin\Controllers\Admin\Layouts->advSettingsShowAction()
#1 application/libraries/Ilch/Page.php(137): Ilch\Page->loadController()
#2 index.php(68): Ilch\Page->loadPage()
#3 {main}
  thrown in application/modules/admin/controllers/admin/Layouts.php on line 330

ToDo:

• ...

Type of change

• Bug fix (non-breaking change which fixes an issue)

This PR has been tested in the following browsers:

• Chrome
• Firefox
• Opera
• Edge