Return 500 error when a resource is not ready

[severo]

related to #404

If a dataset has been created, and the webhook has been triggered, the dataset should be in the queue. Before a worker has completed the creation of the response for the endpoints of this dataset, if a request is received on these endpoints, we currently return 400, telling that the resource does not exist.

It's better to check the content of the queue, and return a 500 error (the server has "failed" to create the resource in time). It allows to separate this case from a request to a non-existent dataset. A 500 error means that the client can retry with the same request later.

Another option would have been to return 200, with a response that includes the state (in progress / done) and the data if any, but it would show internals of the server, and most importantly, would make the client more complicated without a reason.

[severo]

We will implement it only for /splits-next and /first-rows, since /splits and /rows will be deprecated and should remain unchanged until then.

[severo]

see #443