server: use custom error page

server/pages.go

@@ -31,7 +31,8 @@ type Page interface {
 func writePage(p Page, w http.ResponseWriter) {
 	t, err := template.ParseFS(staticFS, "static/templates/" + p.SourceFile())
 	if err != nil {
-		http.Error(w, err.Error(), http.StatusInternalServerError)
+		http.Error(w, "Internal server error: " + err.Error(),
+			http.StatusInternalServerError)
 		return
 	}
 
@@ -41,6 +42,21 @@ func writePage(p Page, w http.ResponseWriter) {
 	}
 }
 
+type ErrorPage struct {
+	Status string
+	Reason string
+}
+
+func MakeErrorPage(status, reason string) (p ErrorPage) {
+	p.Status = status
+	p.Reason = reason
+	return
+}
+
+func (p ErrorPage) SourceFile() string { return "error.html" }
+func (p ErrorPage) Title() string { return "" }
+func (p ErrorPage) Body() template.HTML { return template.HTML("") }
+
 type IndexPage struct {
 	Showcase string
 }