Add UserNotFound Exception 추가

src/main/java/com/example/springsecuritystudy/common/UserNotFoundException.java

@@ -0,0 +1,12 @@
+package com.example.springsecuritystudy.common;
+
+public class UserNotFoundException extends RuntimeException {
+
+	public UserNotFoundException(String message) {
+		super(message);
+	}
+
+	public UserNotFoundException() {
+		super("유저를 찾을 수 없습니다.");
+	}
+}