refactor : (#31) add oauth failerHandler

Author: qkrwndnjs1075

src/main/kotlin/entry/dsm/gitauth/equusgithubauth/global/oauth/handler/CustomOAuth2AuthenticationFailureHandler.kt

@@ -0,0 +1,20 @@
+package entry.dsm.gitauth.equusgithubauth.global.oauth.handler
+
+import jakarta.servlet.http.HttpServletRequest
+import jakarta.servlet.http.HttpServletResponse
+import org.springframework.security.core.AuthenticationException
+import org.springframework.security.web.authentication.AuthenticationFailureHandler
+
+class CustomOAuth2AuthenticationFailureHandler : AuthenticationFailureHandler {
+    override fun onAuthenticationFailure(
+        request: HttpServletRequest,
+        response: HttpServletResponse,
+        exception: AuthenticationException
+    ) {
+        response.contentType = "application/json"
+        response.status = HttpServletResponse.SC_UNAUTHORIZED
+        response.writer.write(
+            """{error: OAuth2 로그인 실패, message: ${exception.message}}"""
+        )
+    }
+}