Workers processes can be started with unavailable ModelVersions

I found the equivalent of #1406 (closed), but with model versions, while testing a random process for !1902 (merged). Nothing stops you from selecting a ModelVersion that is in a created or error state, for a worker that requires a model, and then starting a process. When the task with this model version runs, the Ponos agent marks it as in an error state, as it cannot download the requested file before starting the container (example).

Just like with #1406 (closed), we should not allow building a recipe that includes the URL of a ModelVersion that is not available, and raise a HTTP 400 when trying to start a process in that situation.