Add collection errors.
Replace defer_async with simulate_async so that exceptions raised also happen asynchronously.
Flesh out ObjectNotFound error and add ObjectAlreadyExists error.
Document errors that should be raised by .create().
Raise errors from inmemory collection methods.
Add tests for exception construction.
Update delete missing row test.
Test for attempt to create an object that already exists.
Add test for updating object that doesn't exist.
Add tests for .get().
Tests for simulate_async.
Remove TODO that appears to no longer be relevant.
Handle CollectionUsageError exceptions in request handlers.
Write errors out as JSON.
Add tests for updating and deleting non-existent objects.
Clean up deferred chains so that errbacks are always applied.
Add first tests for CollectionUsageErrors, factor out error checking method and update successful responses.
Tests for CollectionUsageErrors on ElementHandler requests.
Remove ensure_deferred (it's now unused, plus it's a trap for the unwary -- i.e. past me -- because the calculation of the value easily escapes the errback chain).
Fix CollectionObjectAlreadyExists docstring.