raise informative error when selectable can't be extended

raise informative error when selectable can't be extended

An informative error is raised for the use case where
:meth:`.Insert.from_select` is being passed a "compound select" object such
as a UNION, yet the INSERT statement needs to append additional columns to
support Python-side or explicit SQL defaults from the table metadata. In
this case a subquery of the compound object should be passed.

Fixes: #8073
Change-Id: Ic4a5dbf84ec49d2451901be05cb9cf6ae93f02b7