Fix exit code from rebuildLocalisationCache.php

Fix exit code from rebuildLocalisationCache.php

When --threads was used, the exit code was unconditionally 0, even if a
child thread threw an exception, because the exit code was "abused" for
the count of rebuilt languages. So, don't do that. We don't really need
a count of rebuilt languages that badly, just pass through the exit
status of the child.

Bug: T92900
Change-Id: I77922e686fe7da4fc25bb7f579ed70e1b107eaca