fix: error when unconfirmed message not found

[luddd3]

Fixes #152

An error is thrown when a message isn't found in _unconfirmedMessages. I don't have a working example of when it might happen, but when I tested around I could reproduce it with reconnects. This is in line with what michaelfitzhavey's team mates found.

Even if we are unsure how it happens, I think it is fair to say that it is better if it doesn't throw an error, since the error states that the message that we want to remove already is removed =)

[jwalton]

My concern here is, if the message isn't in _unconfirmedMessages, where is it? Is it maybe in _messages, queued to be re-sent? I'm wondering if this is some corner case where we are requeing the messages because the connection dropped, but then some of the messages actually got sent and we got the confirm "late". If this is the case, we'd end up re-sending this message, even though it was already sent. Maybe we want to search for the message in _messages if it's not in _unconfirmedMessages? And error if it's in neither?

There's some underlying bug here, and I don't like it. :P

[luddd3]

I think you might be right. I'm closing this pull request either way since you have rewritten it in TypeScript now. I will create and open another pull request if I find something more tangible.