Return error code 404 instead of 412 when dataset is not found

[josvandervelde]

Description

Minor issue, but 404 is the default for when a resource does not exist.

See for instance https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#client_error_responses
404: "In an API, this can also mean that the endpoint is valid but the resource itself does not exist."

Steps/Code to Reproduce

test.openml.org/api/v1/json/data/9999999999

Expected Results

Error code 404 (Not Found)

Actual Results

Error code 412 (Precondition failed)