Response code for DataSourcesResource on non-existent dataSource or segmentId

#7490 introduced 404 (not found) response in `DataSourcesResource` for non-existent segment ids. As minimum, I think it would be consistent to return 404 instead of 204 (no content) for non-existent segment id for `/{dataSourceName}/segments/{segmentId}` as well.

As maximum, all endpoints in this resources should return 404 on non-existent data source.

There is a [heated discussion on SO](https://stackoverflow.com/q/11746894) regarding whether it's semantically more correct to return 204 or 404 in these situations. To me, it feels that 404 is better.

FYI @dampcake @jon-wei

Start by reading DataSourcesResource and the 404 behavior introduced by #7490, then inspect the endpoint for /{dataSourceName}/segments/{segmentId}. Determine whether the change should cover that endpoint only or all resource endpoints; done means the agreed non-existent data source or segment cases return the expected 404 response instead of 204.

Written by the indexing model from the issue text.