Results 2 issues of Masahiro Nawata

Fix hanging by `KeyboardInterrupt` exception. This hanging occurrs when typing `` after `nvim.call('input', '')`. `KeyboardInterrupt` is not derived from `Exception` class, then no error reply was returned.

When `ssl::stream::async_read_some` is pending, a cancel error, which is occured by `pending_read_`'s cancel, is passed to a handler while some data is read from a SSL instance. So, error code...