Merge pull request #527 from roboflow/iuri/fix-num-errors-undefined

Increment `num_errors` only if `pingback` initialized

inference/core/interfaces/http/http_api.py

@@ -433,7 +433,7 @@ async def count_errors(request: Request, call_next):
                     Response: The response from the next middleware or endpoint.
                 """
                 response = await call_next(request)
-                if response.status_code >= 400:
+                if self.model_manager.pingback and response.status_code >= 400:
                     self.model_manager.num_errors += 1
                 return response