fixed api error responses failing to serialize exceptions

core/src/main/java/eu/mhsl/craftattack/spawn/core/api/server/HttpServer.java

@@ -79,7 +79,7 @@ public class HttpServer {
             try {
                 response = new Response(Response.Status.SUCCESS, null, exec.get());
             } catch(Exception e) {
-                response = new Response(Response.Status.FAILURE, e, null);
+                response = new Response(Response.Status.FAILURE, e.getMessage(), null);
             }
             return HttpServer.this.gson.toJson(response);
         }