Misleading error when trying to rack device from non-racked state when rack face not specified

[Gelob]

Environment

• Python version: Python 3.6.8
• NetBox version: 2.9.7

Steps to Reproduce

1. Create a device and under Location only select the Site and Rack. Leave all other fields empty or null (------). You should now have a non-racked device assigned to the rack.
2. Edit the device and select an available Position in the rack. Click Update
3. Observe error saying the rack unit is not available The lowest-numbered unit occupied by the device Select a valid choice. 16 is not one of the available choices.

Expected Behavior

I expected an error highlighting the rack face field. Similar to changing a racked device's position but nulling out the rack face field. Must specify rack face when defining rack position.

Observed Behavior

Netbox gave an error saying my RU was not available but really it was erroring because I did not select a Rack face now that I am trying to rack the device coming from a non-racked state.

[jeremystretch]

This one is tricky, because the individual form fields are validated prior to calling clean() on the model. This means that the position field is validated against its list of choices prior to checking that both face and position are defined.