Code review comment for lp:~nick-dedekind/mir/trusted_sessions

std::shared_ptr<TrustSession> Shell::start_trust_session_for(std::string& error,
                                                                  std::shared_ptr<Session> const& session,
                                                                  shell::TrustSessionCreationParameters const& params) = 0;

This "error" parameter is not an idiomatic way to handle failure. Simply throwing an exception derived from std::exception that returns the error in what() will result in invoke<>() setting the error.