t/Util_future.t: complete the tests for future_of() function.
Util: implement future_of() function.
Util::future_of(): improve error-reporting with the new Future::Q 0.040.
Main::PSGI::View: add response_error_html() method.
Main::PSGI: properly implement _handle_get_timeline_list().
Main::PSGI: re-implement _handle_tl_{get_statuses,post_statuses,ack} with Future::Q for coding simplicity. "updates" API is not re-written because it is possible for watch_* methods to call the callback multiple times. (If they do, however, it should be a bug because the watcher is explicitly disabled in the callback...)
Main::PSGI: rewrite "updates" API with Future::Q. Their callbacks should be called once anyway...