error reporting: unify error message for `nw.col('a').fill_null(3).over('b')`

[MarcoGorelli]

I noticed in #2443 that this gives a SyntaxError for DuckDB, and is deprecated in pandas

TBH it just doesn't seem like a useful operation, the over serves no purpose here

Not totally sure how yet, but we may want to unify the error message for this operation

Low-prio though, as I can't see why anyone would write this

[MarcoGorelli]

More generally, if an operation is not an aggregation and is not order-dependent, then it probably doesn't make sense to call over on it? Maybe that's the rule we can use to raise here