add error handling (frontend) + exception changes

Co-authored-by: Christoph <tobi@urpost.de>
Co-authored-by: Mahgs <maxoerter@gmx.de>

src/main/java/mops/gruppen2/domain/Exceptions/GroupIdMismatchException.java

@@ -0,0 +1,13 @@
+package mops.gruppen2.domain.Exceptions;
+
+import org.springframework.http.HttpStatus;
+
+public class GroupIdMismatchException extends EventException {
+    public GroupIdMismatchException(String msg) {
+        super("Falsche Gruppe für Event." + "    (" + msg + ")", HttpStatus.INTERNAL_SERVER_ERROR);
+    }
+
+    public GroupIdMismatchException() {
+        super("", HttpStatus.INTERNAL_SERVER_ERROR);
+    }
+}