Unauthenticated users may see a "forever loading" failure state

### What's the problem?
Some users, upon attempting to load ember-twiddle (either viewing an existing twiddle, or creating a new twiddle) see nothing except a big loading spinner that never goes away

### Why does this happen
Unauthenticated users are rate-limited by GitHub, and there does not appear to be appropriate error handling in place to surface this information in the UI

### Who This Affects
Users who end up in a GitHub API rate limited state (e.g., they're on a corporate network with many other users, and have _not logged into twiddle with their github account_).

### How this problem can be significantly mitigated

We could improve the mode of failure so that it tells the user what's going on in this failure state, and presents them with a "next best action" is (e.g., login to github so they're no longer rate limited by IP address).

### Related Tickets
#375