Handle API failure

[ohmoses]

Closes #713

To try out the fix:

• Start Publish and API, open Publish, kill API, navigate to any collection list or document; you should see an error message.
• Restart API and click "Try again" or navigate elsewhere and back; the collection/document loads.
• Kill API and refresh Publish; you get the error message instead of the sign in screen.

Notes/explanations:

• I tried to reuse mechanisms that exist or have existed in the code at some point, e.g. window.__api__ window.__error__ and the api reducer to convey the API error from the backend to the frontend.
• When you get a backend error (backend failed to connect in the first place), the "Try again" button reloads the page so that the backend gets a chance to load the data from API.
• On frontend error (API failure while navigating to a collection/document) the "Try again" button didn't trigger refetch of data—it navigates to the same page it's already on, so the only thing that changes is the React Router location hash key. I refetch the data if the key changes. Edit: As per discussion with @eduardoboucas, the "Try again" button now flags the document or document list as dirty, forcing refetch.
• ErrorView in App.js has to be wrapped in Router to be able to use the Link component.
• I fixed an error where trying to delete documents while the API was down triggered a notification falsely informing of successful deletion.

[eduardoboucas]

🚀

[mingard]

👏