change reporting of missing files

use the new parser flags to report all i/o errors directly.

as a notable side effect, the "WARNING" prefix is gone (even though
it is still treated like that, which is mildly insane to start with).

Change-Id: I084375d5e7a3314ae763795f7c318804a9fb84b6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/b4c0d9166e7a8f5dfa5ca052002a5ab318787aa2)