and what would the error be, pray?

If the publisher did `pubsub.publishJSON` then parse() will return an
object, if it isn't able to, it means that the publisher did
`pubsub.publish`. Maybe a little safeguard could be in place like
prepending $#JSON#$ to json messages, but right here there's no error to
handle, if a message cannot be parsed, it's because it wasn't sent as JSON