Handle TimeoutError in FC2WebSocket._main_loop

[15532th]

Fixing #31.

Hopefully I understood original intent correctly: _try_heartbeat gets called after every received message or if there was no messages for heartbeat_interval.
Downside of the way it is done is the use of two _try_heartbeat calls, but it seems to be the easiest way to avoid sending heartbeat in response to error/end of stream message, when _main_loop raises exception.

Is debug a fitting log level for message about timeout? I used it because heartbeat message itself is debug, but perhaps it should have been trace?

[hizkifw]

Looks good! The double call should be fine as there's a debounce logic in the _try_heartbeat method. I think using debug is good, I'm reserving trace for things that are too verbose for debug such as the raw websocket messages.