QFile::copy: include the QTemporaryFile's error message if it fails

Change-Id: I1bd327aeaf73421a8ec5fffd1560a333fdfea909
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@qt.io>

src/corelib/io/qfile.cpp

@@ -808,7 +808,7 @@ QFile::copy(const QString &newName)
                 if (error) {
                     out.close();
                     close();
-                    d->setError(QFile::CopyError, tr("Cannot open for output"));
+                    d->setError(QFile::CopyError, tr("Cannot open for output: %1").arg(out.errorString()));
                 } else {
                     if (!d->engine()->cloneTo(out.d_func()->engine())) {
                         char block[4096];