ApplyProcessTemplate should fail when a template has an unavailable version

#1406 (closed) might have been caused by a process using a template with a WorkerVersion that was not available. The Great Cleansing induced by #1402 (closed) causes 9/9 templates in prod and 12/17 templates in demo to have some WorkerRuns related to unavailable versions, so when a user tries to start a process from any existing template, this is very likely to occur.

The ApplyProcessTemplate endpoint should raise an HTTP 400 when a template has a version that is not Available or does not have a docker_image.