[misc][distributed] improve libcudart.so finding (#7127)

vllm/distributed/device_communicators/custom_all_reduce_utils.py

@@ -145,6 +145,7 @@ def can_actually_p2p(
     p_tgt.start()
     p_src.join()
     p_tgt.join()
+    assert p_src.exitcode == 0 and p_tgt.exitcode == 0
     result: List[bool] = []
     for src, tgt in zip(batch_src, batch_tgt):
         a = result_queue.get()
@@ -221,7 +222,8 @@ def gpu_p2p_access_check(src: int, tgt: int) -> bool:
             # wrap raised exception to provide more information
             raise RuntimeError(
                 f"Error happened when batch testing "
-                f"peer-to-peer access from {batch_src} to {batch_tgt}") from e
+                f"peer-to-peer access from {batch_src} to {batch_tgt}:\n"
+                f"{returned.stderr.decode()}") from e
         result = pickle.loads(returned.stdout)
         for _i, _j, r in zip(batch_src, batch_tgt, result):
             cache[f"{_i}->{_j}"] = r