Cause an HTTP 400 when the user requests GPUs but no workers support them

If the user uses use_gpu=True when starting a process, but there are no worker runs whose versions have gpu_usage != disabled, the StartProcess endpoint should cause an HTTP 400 error with an explicit message.

If there is at least one worker run that supports GPUs, just do not require GPUs for the others and start normally.

This will make it much more explicit, especially when developing a worker (a situation where you would often be running a worker all alone), that even though you asked for a GPU, there won't be any.