feat: create global exception handler

src/main/java/com/runningmate/backend/GlobalExceptionHandler.java

@@ -0,0 +1,19 @@
+package com.runningmate.backend;
+
+import com.runningmate.backend.member.exception.FieldExistsException;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+
+import java.util.List;
+import java.util.Map;
+
+@RestControllerAdvice
+public class GlobalExceptionHandler {
+
+    @ExceptionHandler(FieldExistsException.class)
+    public ResponseEntity<Map<String, List<String>>> handleFieldExistsException(FieldExistsException fe) {
+        return ResponseEntity.status(HttpStatus.CONFLICT).body(Map.of("exists", fe.getExists()));
+    }
+}