Use JSON for errors and page response

error.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"fmt"
+	"net/http"
+)
+
+type Error struct {
+	Message string `json:"message"`
+}
+
+func sendError(w http.ResponseWriter, code int, msg string, args ...any) {
+	w.WriteHeader(code)
+	sendJSON(w, Error{Message: fmt.Sprintf(msg, args...)})
+}