Change file_uris() to error if without_files was set

[wjones127]

Description

You can instantiate a DeltaTable struct without loading the files to save memory. If you call .file_uris() on that table, it will return an empty iterator, which isn't quite correct. I fear this might lead to issues later, where we can't differentiate between an empty table and one that was loaded without files. Perhaps this should return an error instead?

Use Case

Related Issue(s)

Found while looking at #866.