FileSource::create_file_opener takes a Result argument that is never used

[AdamGS]

Seems like somewhere in the shuffle of #14224, FileSource::create_file_opener got a Result argument which isn't very idiomatic IMO, especially as it doesn't seem like there's any special handling of it anywhere.

Another small point here is that the function itself returns a Result that doesn't seem to actually propagate any error (JSON and Parquet even panic), but I can imagine that some FileSource implementation in the future would like the ability to do that.
I have a Vortex PR that attempts to track the datafusion upstream and experiment with the new APIs, and while we do error there I think making those errors into panics or validating those invariants somewhere else would also make sense.