when qwen rerank model not return ok, raise exception to notice user (#3593)

### What problem does this PR solve?

When calling the Qwen rerank model, if the model does not return
correctly, an exception should be raised to notify the user, rather than
simply returning a value of 0, as this would be confusing to the user.
### Type of change          

- [x] New Feature (non-breaking change which adds functionality)

rag/llm/rerank_model.py

@@ -452,4 +452,5 @@ class QWenRerank(Base):
             for r in resp.output.results:
                 rank[r.index] = r.relevance_score
             return rank, resp.usage.total_tokens
-        return rank, 0
+        else:
+            raise ValueError(f"Error calling QWenRerank model {self.model_name}: {resp.status_code} - {resp.text}")