Display a better error message if an event can't be remotely cancelled

[MelissaAutumn]

(From discussion at #768)

We should create and show a new error message if a user denies a booking, but for some reason our system can't remove the HOLD event. (Whether they've already removed it on their end, or there's an api error removing it.)

Right now it removes the appointment from the Bookings tab but displays the invalid link error.