[TYPO3-core] RFC #11799: Use typo3printError() if backend is locked

[Steffen Gebert]

Hi!

This is a SVN patch request.

Type: bugfix

Branches: trunk

BT reference:
http://bugs.typo3.org/view.php?id=11799

Problem:
You can create a file called typo3conf/LOCK_BACKEND to disable all BE  
access.

If this file exists, TYPO3 will die("'Browser backend is locked for  
maintenance. Remove lock by removing the file "typo3conf/LOCK_BACKEND" or  
use CLI-scripts").

Solution:
make this nice-looking using typo3printError()

Regards
Steffen