Improve error msg when an INSERT INTO ... SELECT ... uses a source

[ggevay]

It's not allowed for an INSERT INTO ... SELECT ... to (transitively) refer to any sources. The error msg is currently:

Error: invalid selection: operation may only refer to user-defined tables

It would be good to clarify:

• This is a limitation of INSERT INTO ... SELECT ... queries. (and maybe other similar things, e.g., some UPDATE?)
• Which are the problematic objects.
• The operation may only refer to user-defined tables is not so clear: it's actually ok to refer to views, mat views, etc., but the final transitive dependencies at the end are what count.

A confused user:
https://materializeinc.slack.com/archives/C053EPHMU05/p1715181840656969?thread_ts=1715181618.113209&cid=C053EPHMU05