fix: 중복 이메일 가입 에러 핸들링 추가

Author: m3k0813

src/main/java/com/gotogether/global/oauth/exception/DuplicatedEmailException.java

@@ -0,0 +1,9 @@
+package com.gotogether.global.oauth.exception;
+
+import org.springframework.security.core.AuthenticationException;
+
+public class DuplicatedEmailException extends AuthenticationException {
+	public DuplicatedEmailException(String msg) {
+		super(msg);
+	}
+}

src/main/java/com/gotogether/global/oauth/handler/CustomFailureHandler.java

@@ -6,10 +6,10 @@
 import org.springframework.security.web.authentication.AuthenticationFailureHandler;
 import org.springframework.stereotype.Component;
 
-import com.gotogether.global.oauth.util.ErrorResponseUtil;
 import com.gotogether.global.apipayload.code.status.ErrorStatus;
+import com.gotogether.global.oauth.exception.DuplicatedEmailException;
+import com.gotogether.global.oauth.util.ErrorResponseUtil;
 
-import jakarta.servlet.ServletException;
 import jakarta.servlet.http.HttpServletRequest;
 import jakarta.servlet.http.HttpServletResponse;
 
@@ -18,8 +18,12 @@ public class CustomFailureHandler implements AuthenticationFailureHandler {
 
 	@Override
 	public void onAuthenticationFailure(HttpServletRequest request, HttpServletResponse response,
-		AuthenticationException exception) throws IOException, ServletException {
+		AuthenticationException exception) throws IOException {
 
-		ErrorResponseUtil.sendErrorResponse(response, ErrorStatus._INTERNAL_SERVER_ERROR);
+		if (exception instanceof DuplicatedEmailException) {
+			ErrorResponseUtil.sendErrorResponse(response, ErrorStatus._USER_EMAIL_ALREADY_EXISTS);
+		} else {
+			ErrorResponseUtil.sendErrorResponse(response, ErrorStatus._INTERNAL_SERVER_ERROR);
+		}
 	}
 }

src/main/java/com/gotogether/global/oauth/service/CustomOAuth2UserService.java

@@ -19,6 +19,7 @@
 import com.gotogether.global.oauth.dto.KakaoResponseDTO;
 import com.gotogether.global.oauth.dto.OAuth2Response;
 import com.gotogether.global.oauth.dto.TokenDTO;
+import com.gotogether.global.oauth.exception.DuplicatedEmailException;
 import com.gotogether.global.oauth.util.JWTUtil;
 import com.gotogether.global.util.CookieUtil;
 
@@ -64,7 +65,7 @@ public OAuth2User loadUser(OAuth2UserRequest userRequest) throws OAuth2Authentic
 
 			Optional<User> duplicateEmail = userRepository.findByEmail(oAuth2Response.getEmail());
 			if (duplicateEmail.isPresent()) {
-				throw new GeneralException(ErrorStatus._USER_EMAIL_ALREADY_EXISTS);
+				throw new DuplicatedEmailException("이미 가입된 이메일입니다.");
 			}
 
 			User userEntity = User.builder()