GLTFIO: check return value of QFile::open()

We checked that the file exists prior to opening it, but
it could still be unreadable (due to permissions, e.g.),
so check with QFile::open() and emit QFile::errorString()
in case it goes wrong.

Coverity-Id: 161328
Change-Id: I3489488023bb697d6cb9eee6be07c0edd923c478
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>