message: Improve the error message of container not found.
diff: @@ -160,7 +160,9 @@ Status ResourceMgr::DoLookup(const string& container, TypeIndex type,
  tf_shared_lock l(mu_);
  const Container* b = gtl::FindPtrOrNull(containers_, container);
  if (b == nullptr) {
  - return errors::NotFound("Container ", container, " does not exist.");
  + return errors::NotFound("Container ", container,
  + " does not exist. (Could not find resource: ",
  + container, "/", name, ")");
  }
  auto r = gtl::FindPtrOrNull(*b, {type.hash_code(), name});
  if (r == nullptr) {
  