Fix error message pass through in API for events and drafts.

Summary:
I'm not sure how we ended up using `e.message` in the first place, but
as far as I can tell, it does not work. For example, try using an invalid
calendar ID when creating an event. You'll get a blank error message,
which makes debugging the request being sent very difficult.

I did not verify that this is the case for all of these API endpoints (I
just verified a couple of the events ones), but the code paths all look
similar to me.

Test Plan:
vagrant@precise64:/vagrant$ curl -X POST http://localhost:5555/n/dl75gys73g7t5ujkheyfauo2b/events -d '{ "calendar_id": "foobar", "title": "Friday meeting", "when": {"start_time": "1417233600", "end_time": "1417237200" }, "location": "Inbox HQ", "participants": [ { "email": "spang@inboxapp.com", "name": "Christine Spang" } ] }'
{
  "message": "Invalid calendar public id foobar",
    "type": "invalid_request_error"

whereas "message" was blank before, etc.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D784