failed finalize should be considered error

In order to use makensis for unattended scripts it's important to evaluate the error code:
http://forums.winamp.com/showthread.php?t=252342

Unfortunately a failed "finalize" is not considered an error by makensis and it still returns %errorlevel% 0, e.g.:
!finalize 'C:\SignCode.cmd "%1"'

This should be at least a warning, so that scripts using "makensis" /WX" can detect a failed installer compilation.

Tested with NSIS 3.0rc2