QtIFW

Check return value of ShellExecuteExW instead of GetLastError

It's unclear from the MSDN documentation whether LastError is also
set if ShellExecuteEx succeeds. Therefore rather check for the
return value.

Task-number: QTBUG-31788
Change-Id: Id74377445c5e64a056d4f1c0ec6ae29b02a0d98e
Reviewed-by: Karsten Heimrich <karsten.heimrich@digia.com>