Actions are only scheduled for a valid account, api returns an HTTP 403/503 error otherwise.
Fixes T420.

Summary:
We check account.sync_state to determine validity.
403: invalid
503: connerror

Test Plan:
Test added to tests/api/test_validation.py

Reviewers:
emfree

Reviewed By:
emfree

Subscribers:
charles

Maniphest Tasks:
T420

Differential Revision:
https://review.inboxapp.com/D451