handle ERR_NOSUCHNICK when /msg-ing someone that doesn't exist

```
> /msg fakename hi hello there
{cmd: "ERR_NOSUCHNICK", message: "No such nick or channel name"}
```

we receive that error but the app doesn't do anything with it. it should remove that entry from the convo list and message you about it.

Start by tracing where ERR_NOSUCHNICK responses are handled and where the conversation list is updated. Confirm the existing notification path for server errors. Done means the nonexistent nick is removed from the conversation list and the user is informed.

Written by the indexing model from the issue text.