Avoid UnicodeDecodeError from twisted

[jvperrin]

Twisted currently gives an error like this when something like an emoji is cut off part of the way through a line: https://i.fluffy.cc/fBt1HhSG7NPJ9SWMVPzVzjjvCDRW8dKv.html

Although this just means slackbridge restarts (which isn't terrible), we should try and avoid it if possible.