map CachedArtifactNotFoundError to PreviousStepStillProcessingError
Part of https://github.com/huggingface/datasets-server/issues/1443
Our first cache error_code is `UnexpectedError` (4558 records) caused by `CachedArtifactNotFoundError`. This happens when a step runs because one of its predecessors is ready but the others are still waiting or being processed.
Mapping `CachedArtifactNotFoundError` to a custom cacheable error `PreviousStepStillProcessingError` so that, we have a clear understanding. 

map CachedArtifactNotFoundError to PreviousStepStillProcessingError: Part of https://github.com/huggingface/datasets-server/issues/1443
Our first cache error_code is `UnexpectedError` (4558 records) caused by `CachedArtifactNotFoundError`. This happens when a step runs because one of its predecessors is ready but the others are still waiting or being processed.
Mapping `CachedArtifactNotFoundError` to a custom cacheable error `PreviousStepStillProcessingError` so that, we have a clear understanding. 
