Document error boundary behavior

[MattDodsonEnglish]

Jarrah's review in #121 revealed there are a number of particulars regarding error boundaries. These should be documented.

Besides behavior, we also need to document what type of GO land errors would result in the event. Probably could be added to the reference.

[Jarrah-libremfg]

Besides behavior, we also need to document what type of GO land errors would result in the event. Probably could be added to the reference.

Any Go error returned from a service task or call activity will result in an error boundary event being triggered if attached to the node. There's absolutely no filtering, so long as it's one of those nodes.

Probably also worth noting that you could attach an error boundary to a non-service task and the UI would look functional, but it would never be called.