add error handling of github api errors

[nosovk]

Time to time GitHub API can go down, and make responses with 5xx codes and body like that:

{
   "data": null,
   "errors":[
      {
         "message":"Something went wrong while executing your query. This may be the result of a timeout, or it could be a GitHub bug. Please include `DF4A:6E1B:1D670F9:1DAA64D:64E7DC75` when reporting this issue."
      }
   ]
}

It would be nice to show some error modal with "Retry" button to retry the request.
No errors silently ignored :(

[kyoshino]

Netlify CMS has a background service status check and shows an alert when the Git backend is down.

decaporg/decap-cms#3889

I think we need this and took a screenshot before:

The English label is

The backend service is experiencing an outage. See %{details} for more information