cleanup: use ErrorToStringOrOK func in other tests that returns nil (#701)

Reference issue: https://github.com/ooni/probe/issues/2040

internal/engine/experiment/webconnectivity/control.go

@@ -64,7 +64,7 @@ func Control(
 	if err != nil {
 		err = netxlite.NewTopLevelGenericErrWrapper(err)
 	}
-	sess.Logger().Infof("control for %s... %+v", creq.HTTPRequest, err)
+	sess.Logger().Infof("control for %s... %+v", creq.HTTPRequest, model.ErrorToStringOrOK(err))
 	(&out.DNS).FillASNs(sess)
 	return
 }

internal/engine/experiment/webconnectivity/dnslookup.go

@@ -44,7 +44,7 @@ func DNSLookup(ctx context.Context, config DNSLookupConfig) (out DNSLookupResult
 			}
 		}
 	}
-	config.Session.Logger().Infof("%s... %+v", target, err)
+	config.Session.Logger().Infof("%s... %+v", target, model.ErrorToStringOrOK(err))
 	out.Failure = result.Failure
 	out.TestKeys = result
 	return

internal/engine/experiment/webconnectivity/httpget.go

@@ -63,7 +63,7 @@ func HTTPGet(ctx context.Context, config HTTPGetConfig) (out HTTPGetResult) {
 		Session: config.Session,
 		Target:  target,
 	}.Get(ctx)
-	config.Session.Logger().Infof("GET %s... %+v", target, err)
+	config.Session.Logger().Infof("GET %s... %+v", target, model.ErrorToStringOrOK(err))
 	out.Failure = result.Failure
 	out.TestKeys = result
 	return