generateLocalAutoload.php: Abort for web requests

generateLocalAutoload.php: Abort for web requests

The Maintenance class already does this, though this script does not use it
for the obvious reason that when there is a missing or incorrect autoloader
entry, it may not be possible to initialize MediaWiki.

Hence it is necessary to do that explicitly in this script. Overwriting
autoload.php potentially could disrupt the operation of a production wiki
(the file is truncated when it is opened).

Change-Id: I05d5d4b7c30c4643e71ca5819783627688e15da7