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

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

internal/engine/experiment/hirl/hirl.go

@@ -131,7 +131,7 @@ func (m Measurer) Run(
 		tk.Tampering = (tk.Tampering || result.Tampering)
 		completed++
 		percentage := (float64(completed)/float64(len(m.Methods)))*0.5 + 0.5
-		callbacks.OnProgress(percentage, fmt.Sprintf("%s... %+v", result.Name, result.Err))
+		callbacks.OnProgress(percentage, fmt.Sprintf("%s... %+v", result.Name, model.ErrorToStringOrOK(result.Err)))
 		if completed >= len(m.Methods) {
 			break
 		}

internal/engine/experiment/urlgetter/multi.go

@@ -106,7 +106,7 @@ func (m Multi) collect(expect int, overallStartIndex int, overallCount int, pref
 		count++
 		percentage := float64(count) / float64(overallCount)
 		callbacks.OnProgress(percentage, fmt.Sprintf(
-			"%s: measure %s: %+v", prefix, entry.Input.Target, entry.Err,
+			"%s: measure %s: %+v", prefix, entry.Input.Target, model.ErrorToStringOrOK(entry.Err),
 		))
 		outputch <- entry
 	}

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