QtIFW

Fix install fail if there are missing repositories

If any of the repositories was not found or had unparsable
Updates.xml, install failed although there was other valid
repositories. Fixed so that warning is printed when such repositories
found but continue install if there are other valid repositories.

Change-Id: I04fe8d91382a3ea97ce22cd64159a6a7d825128d
Reviewed-by: Iikka Eklund <iikka.eklund@qt.io>