helpers: Fix panic with invalid defaultMarkdownHandler

Fixes #9968

hugolib/config_test.go

@@ -756,3 +756,29 @@ theme_param="themevalue2"
 	})
 
 }
+
+func TestInvalidDefaultMarkdownHandler(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+[markup]
+defaultMarkdownHandler = 'blackfriday'
+-- content/_index.md --
+## Foo
+-- layouts/index.html --
+{{ .Content }}
+
+`
+
+	b, err := NewIntegrationTestBuilder(
+		IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+		},
+	).BuildE()
+
+	b.Assert(err, qt.IsNotNil)
+	b.Assert(err.Error(), qt.Contains, "Configured defaultMarkdownHandler \"blackfriday\" not found. Did you mean to use goldmark? Blackfriday was removed in Hugo v0.100.0.")
+
+}