Improve internal logical replication error for missing equality strategy

Improve internal logical replication error for missing equality strategy

This "shouldn't happen", except right now it can with a temporal gist
index (to be fixed soon), because of missing gist support in
get_equal_strategy_number().  But right now, the error is not caught
right away, but instead you get the subsequent error about a "missing
operator 0".  This makes the error more accurate.

Author: Paul Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+renyUApHgSZF9-nd-a0+OPGharLQLO=mDHcY4_qQ0+noCUVg@mail.gmail.com