[vllm] fix: catch exception of vllm async engine (#3789)

### What does this PR do?

vllm async engine error is not catched, which will cause the whole async
loop keep waiting results and hang.

this pr fix the this issue by send the picked error by zeromq, and raise
if received.




| before | after |
| --- | --- |
| <img width="2148" height="1308" alt="image"
src="https://github.com/user-attachments/assets/68f565ab-ffa5-4d52-84d6-3e7a2a7cf16a"
/>| <img width="2278" height="1278" alt="image"
src="https://github.com/user-attachments/assets/ce2a647d-3d18-4f57-ad40-054ea97e87d7"
/> |


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---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

verl/workers/rollout/vllm_rollout/vllm_async_server.py

@@ -100,6 +100,10 @@ class ExternalZeroMQDistributedExecutor(Executor):
         outputs = []
         for socket in self.sockets:
             outputs.append(pickle.loads(socket.recv()))
+
+        for output in outputs:
+            if isinstance(output, Exception):
+                raise output
         return outputs
 
     def check_health(self):

verl/workers/rollout/vllm_rollout/vllm_rollout_spmd.py

@@ -536,7 +536,8 @@ class vLLMAsyncRollout(BaseRollout):
                 await self.socket.send(pickle.dumps(result))
             except Exception as e:
                 logger.exception(f"vLLMAsyncRollout _loop_forever error: {e}")
-                os._exit(-1)
+                await self.socket.send(pickle.dumps(e))
+                break
 
     def _init_worker(self, all_kwargs: list[dict[str, Any]]):
         """Initialize worker engine."""