http: Add an HTTP redirect senteniel error

http/handler.go

@@ -179,6 +179,12 @@ func (this *Handler) serveDocument (
 		this.serveError (
 			res, req,
 			httpError.Status, httpError.Message, false)
+		return
+	}
+	var httpRedirect Redirect
+	if errors.As(err, &httpRedirect) {
+		http.Redirect(res, req, httpRedirect.Location, httpRedirect.Status)
+		return
 	}
 	if err != nil {
 		this.serveError (

http/http.go

@@ -22,6 +22,19 @@ func (err Error) Error () string {
 	}
 }
 
+// Redirect, if returned from a template, will cause the server to redirect the
+// client to the given location.
+type Redirect struct {
+	Status int
+	Location string
+}
+
+func (err Redirect) Error () string {
+	return fmt.Sprintf (
+		"%d: %s: %s",
+		err.Status, http.StatusText(err.Status), err.Location)
+}
+
 // HTTPData represents information about an ongoing HTTP request that is made
 // available to templates as they are being executed.
 type HTTPData struct {