Add onOffline/onError callbacks in serviceWorker.js (next)

[natrim]

Hello,

i added onOffline and onError callback to the serviceWorker.js

They are useful ie. to display user message when in offline
and ie. to log the error on serviceWorker register fail.

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[Timer]

Hey! Thanks for this but we're currently switching to Workbox and are going to make service workers super configurable, so this change is out of scope for now.

[natrim]

the 2.0 release has still the old serviceWorker.js though? so this PR should still be relevant