Fix error code handling for scheduling API
This MR makes the appropriate adjustments to handle the changes from the steering!22 (merged) merge request, namely the renamed attribute names in scheduling requests and the HTTP 409 status codes sent for schedule conflict responses.
This MR is based on !9 (merged) and !10 (merged) which should be merged before.
Fixes #93 (closed).
Edited by Konrad Mohrfeldt