message: poplar_executor: Improve error message when outfeed is missing
  Test Plan: Existing tests pass.
  Reviewers: #tensorflow, georgep
diff: @@ -533,8 +533,14 @@ void PoplarExecutor::ConnectOutfeedToStreamCallback(
  }
  for (const auto& outfeed_info : outfeed_infos) {
  - auto* outfeed_context =
  - outfeed_contexts_.at(outfeed_info.config.feed_id()).get();
  + const auto& outfeed_id = outfeed_info.config.feed_id();
  + auto itr = outfeed_contexts_.find(outfeed_id);
  + if (itr == outfeed_contexts_.end()) {
  + LOG(FATAL) << "Outfeed with id='" << outfeed_id
  + << "' is not registered, but is required by the engine.";
  + }
  +
  + auto* outfeed_context = itr->second.get();
  auto tensor_count = outfeed_context->shapes.size();
  for (unsigned j = 0; j < tensor_count; ++j) {
  size_t length = ShapeUtil::ByteSizeOf(outfeed_context->shapes[j]);
  