Handle response errors

[cbdm]

Currently there are no checks to validate if the response we got from the API is valid, so if the server is down or we have exceeded the daily quota, the behavior is undefined (e.g., we might silently fail, return a messed up calendar, show the user an exception). We should probably handle this in a more elegant manner, e.g., alert whether we don't have quota / the server is down and they should retry in X hours.