JS models loading state and error handling consistency

[noirbizarre]

This PR ensure that every JS model:

• set the loading flag to true on fetch/save/update (and so widgets properly display a spinner)
• save/update methods always accept an optional error handler
• save/update methods always handle errors, even when no error handler is provided

The provided default error handler ensures:

• the loading state is set back to false (prevent infinite spinning on widgets)
• in the absence of a provided error handler that:
  • the error is submitted to Sentry if necessary (when there was no server side error handled by Sentry) and so notified to the user using the global Sentry handler
  • the error is logged as error in the JS console as fallback

To improve the console fallback, a feedback pubsub needs to be implemented allowing views to listen on model errors and then notify the user.

This implements/fixes #2245 and ensures datagouv/data.gouv.fr#133 fix (#2246) does not get stuck on the spinner instead of properly display the form error