message: Add descriptive error message in FunctionLibraryRuntimeImpl
diff: @@ -1103,7 +1103,7 @@ void FunctionLibraryRuntimeImpl::Run(const Options& opts, Handle handle,
  std::vector<Tensor>* rets,
  DoneCallback done) {
  if (opts.cancellation_manager && opts.cancellation_manager->IsCancelled()) {
  - done(errors::Cancelled(""));
  + done(errors::Cancelled("Function was cancelled before it was started"));
  return;
  }
  Options run_opts = opts;
  