Let qtpaths return a failure to the caller if it found nothing

Otherwise, the only hint that we get that nothing was found is an empty
line. You could detect that from a shell by assigning to a variable and
checking if it's empty (provided $IFS is set to a newline). But a return
value is much easier.

Change-Id: I00608be8a23a487c9f90a4080479c8f527a8eb3f
Reviewed-by: David Faure <david.faure@kdab.com>