16723: Reject invalid updates in API stub.

16723: Reject invalid updates in API stub.

When dispatcher notices crunch-run has exited and the last known state
is Locked, it requeues the container. If crunch-run changed the
container state to Running before exiting (and dispatcher hasn't
noticed yet), dispatcher relies on RailsAPI/controller to reject the
requeue attempt.

Without this, the scheduler's state=Queued call was being accepted
even after losing a race to the stub's state=Running call,
occasionally causing a container to run twice and fail the randomized
simulation test.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@tomclegg.ca>