message: Make NcclManager's error message more descriptive.
  This should make it easier to tracking down NCCL/CUDA errors in NcclManager.
diff: @@ -48,7 +48,7 @@ int NcclManager::instance_count = 0;
  do { \
  ncclResult_t nccl_status = (__VA_ARGS__); \
  if (nccl_status != ncclSuccess) { \
  - return errors::Internal(ncclGetErrorString(nccl_status)); \
  + return errors::Internal("NCCL: ", ncclGetErrorString(nccl_status)); \
  } \
  } while (0)
  @@ -56,7 +56,7 @@ int NcclManager::instance_count = 0;
  do { \
  cudaError_t cuda_status = (__VA_ARGS__); \
  if (cuda_status != cudaSuccess) { \
  - return errors::Internal(cudaGetErrorString(cuda_status)); \
  + return errors::Internal("CUDA: ", cudaGetErrorString(cuda_status)); \
  } \
  } while (0)
  