[ADD]エラー画面処理を実装しました

src/main/java/com/example/demo/app/presentation/controller/ErrorController.java

@@ -0,0 +1,12 @@
+package com.example.demo.app.presentation.controller;
+
+import org.springframework.stereotype.Controller;
+import org.springframework.web.bind.annotation.GetMapping;
+
+@Controller
+public class ErrorController {
+	    @GetMapping("/error")
+	    public String showError() {
+	        return "error";
+	    }
+}

src/main/resources/templates/error.html

@@ -0,0 +1,32 @@
+<!DOCTYPE html>
+<html xmlns:th="http://www.thymeleaf.org">
+<head>
+<meta charset="UTF-8" name="viewport" content="width=device-width, initial-scale=1">
+<title>エラー</title>
+<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.7/dist/css/bootstrap.min.css" rel="stylesheet">
+</head>
+<body>
+
+	</head>
+	<body>
+
+	    <div th:replace="~{header :: header}">
+	    </div>
+
+	    <div class="container text-center mt-5">
+
+	        <h2 class="text-danger">
+	            エラーが発生しました
+	        </h2>
+
+	        <p th:text="${errorMessage}"></p>
+
+	        <a class="btn btn-primary" th:href="@{/login}">
+	            ログイン画面へ
+	        </a>
+
+	    </div>
+	
+	<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.7/dist/js/bootstrap.bundle.min.js"></script>
+</body>
+</html>