Improve OBS socket handling

[PeterJCLaw]

Currently failures are handled badly:

• on start if OBS isn't running you get a big stack trace; would be nice to have a cleaner error message
• on Ctrl + C you get a big stack trace around socket errors

In general the OBS web socket is not nicely handled -- perhaps deserves a context manager? Not sure how to integrate that with how the rest of mixtape works though.