message: Work around a gcc 4.9 bug
diff: @@ -335,7 +335,7 @@ void CollectionNotifier::deliver_error(std::exception_ptr error)
  m_error = true;
  m_callback_count = m_callbacks.size();
  - for_each_callback([&](auto& lock, auto& callback) {
  + for_each_callback([this, &error](auto& lock, auto& callback) {
  // acquire a local reference to the callback so that removing the
  // callback from within it can't result in a dangling pointer
  auto cb = std::move(callback.fn);
  @@ -344,7 +344,7 @@ void CollectionNotifier::deliver_error(std::exception_ptr error)
  cb.error(error);
  // We never want to call the callback again after this, so just remove it
  - remove_callback(token);
  + this->remove_callback(token);
  });
  }
  