Refactor Controllers to improve UI

 - Replacing recently added thrown Exceptions with appropriate return/redirect and information to user.
 - Modifying docblocks where appropriate
 - Replacing deprecated functions where possible.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| Refs tickets  | #1413
| License       | MIT
| Doc PR        | -

 - [x] Admin
 - [x] Blocks
 - [x] Categories
 - [x] Extensions
 - [x] Groups
 - [x] Mailer
 - [x] PageLock
 - [x] Permissions
 - [x] Search
 - [x] SecurityCenter
 - [x] Settings
 - [x] Theme
 - [x] Users
 - [x] refactor LogUtil::registerError() to $this->request->getSession()->getFlashbag()->add('error', $msg);

could really use some help with this... @cmfcmf @rojblake