Don't allow \ in installpath on Unix

Also now only show the actual matching character,
not the whole RegExp.

Task-number: QTIFW-512
Change-Id: I179b994b5ee5d7f19b9724d13b2c192a6af01448
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>