Error swallowing in auth.interceptor.ts

https://github.com/google/peoplemath/blob/7e6d8a6695e1f9aa39cb25375a41056c8f627694/src/app/services/auth.interceptor.ts#L61

All non authentication errors are swallowed by the code above.

I think it needs to be changed to
```JavaScript
return throwError(err);
```
because
```JavaScript
of(err)
```
'eats' the error context, which bypass error checking in all layers above and catchError code, stops working.