This is the real change in this PR, but I've convinced myself it's not actually a change.

`ResourceNotFound ` here referred to `restkit.errors.ResourceNotFound`, which is _not_ the same error as `couchdbkit.exceptions.ResourceNotFound` (nor is one a subclass of the other, although they share a common parent), which is what `Document.get` raises.

https://github.com/dimagi/couchforms/commit/4c54a5311ffcbfeae83e4a35977ee4bf1c5a400a#diff-73058831d969485842825596f9aac109R46