[reactos setup] Show critical error if 'txtsetup.sif' cannot be found

If the file txtsetup.sif cannot be found, and the setup is not in unattended mode, show an error and exit the setup application. Without the file, the setup application shows a lot of empty controls that makes the setup unusable. Also, change the icon to give a better visual clue to the user as to what the application does exactly. The icon itself is taken from the shell32 icon folder.