[Misc] Missing error message for custom ops import (#5282)

vllm/_custom_ops.py

@@ -5,8 +5,10 @@ import torch
 try:
     from vllm._C import cache_ops as vllm_cache_ops
     from vllm._C import ops as vllm_ops
-except ImportError:
-    pass
+except ImportError as e:
+    from vllm.logger import init_logger
+    logger = init_logger(__name__)
+    logger.warning("Failed to import from vllm._C with %r", e)
 
 
 # activation ops