Fix strange error message.

Task-number: QTIFW-309

If a component script file is empty or misses the component class
constructor, bail out during binary create or installer execution.

Change-Id: I422c64186652a52122d54b5b042139803c24ba10
Reviewed-by: Niels Weber <niels.weber@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>