Better describe error conditions in spec

[jkarlin]

As requested by TAG review (w3ctag/design-reviews#95), we need to better describe the error conditions.

The document doesn't enumerate its error conditions; it would be nice if this was spelled out, or at least shown more in example code.

E.g., consider the situation when a service worker is woken up to do background sync, and inside that you register for another background sync; what are the implications -- should a UA allow that? How should they notify the user? Are there normative limitations, and if not, are there heuristics?