Change RawMessage::unpack so that it doesn't panic on invalid messages

[eminence]

In RawMessage::unpack, we are doing a bunch of asserts to make sure that the incoming bytes are expected. These asserts will panic, which is not great. Instead, we should convert these assertions into normal if checks that return errors

[eminence]

If anyone wants to work on this, let me know. It could be a good first issue to get familiar with the code. I'll probably tackle this myself in about a week, to chime in if you want to work on it