fix(nettests): failing to start a nettest is a warning

This diff backports #1373 to the release/3.19 branch.

Emitting an error causes ooni/probe-desktop to try to generate a warning
window that (1) is a bit unpleasant to visually see and (2) more
importantly is not proper to show, given that we have default-disabled
experiments right now.

Also, in my book, we should emit warning if we continue running and
errors when we stop. (Maybe it's not the best book considering that
assumes that all errors are fatal; in any case, try to be consistent
with this convention.)

See ooni/probe#2576 for extra information.

cmd/ooniprobe/internal/nettests/run.go

@@ -78,7 +78,7 @@ func RunGroup(config RunGroupConfig) error {
 		return err
 	}
 	if err := sess.MaybeLookupBackends(); err != nil {
-		log.WithError(err).Warn("Failed to discover OONI backends")
+		log.WithError(err).Errorf("Failed to discover OONI backends")
 		return err
 	}
 
@@ -116,7 +116,9 @@ func RunGroup(config RunGroupConfig) error {
 		ctl.RunType = config.RunType
 		ctl.SetNettestIndex(i, len(group.Nettests))
 		if err = nt.Run(ctl); err != nil {
-			log.WithError(err).Errorf("Failed to run %s", group.Label)
+			// We used to emit an error here, now we emit a warning--the proper choice
+			// given that we continue running. See https://github.com/ooni/probe/issues/2576.
+			log.WithError(err).Warnf("Failed to run %s", group.Label)
 		}
 	}