message: Raise a better error message when a DescriptorPool lookup fails because of a missing BUILD dep.
diff: @@ -68,14 +68,18 @@ Status GetDescriptorPoolFromBinary(
  const string& source,
  std::unique_ptr<protobuf::DescriptorPool>* owned_desc_pool) {
  if (!absl::StartsWith(source, "bytes://")) {
  - return errors::InvalidArgument(
  - "Source does not represent serialized file descriptor set proto.");
  + return errors::InvalidArgument(absl::StrCat(
  + "Source does not represent serialized file descriptor set proto. ",
  + "This may be due to a missing dependency on the file containing ",
  + "REGISTER_DESCRIPTOR_POOL(\"", source, "\", ...);"));
  }
  // Parse the FileDescriptorSet.
  protobuf::FileDescriptorSet proto;
  if (!proto.ParseFromString(string(absl::StripPrefix(source, "bytes://")))) {
  - return errors::InvalidArgument(
  - "Source does not represent serialized file descriptor set proto.");
  + return errors::InvalidArgument(absl::StrCat(
  + "Source does not represent serialized file descriptor set proto. ",
  + "This may be due to a missing dependency on the file containing ",
  + "REGISTER_DESCRIPTOR_POOL(\"", source, "\", ...);"));
  }
  return CreatePoolFromSet(proto, owned_desc_pool);
  }
  