Make error message more verbose in QML test test_loadProgress

If the verify(loadProgressMin <= loadProgress) fails, the output was
not verbose enough. The additional outputs will tell us more about
what caused the test to fail.

Task-number: QTBUG-61815
Change-Id: Ia9b45ef4f09a5b25a344e423a7b0c61d5d430542
Reviewed-by: Kai Koehne <kai.koehne@qt.io>