Back-patch "Add parent table name in an error in reorderbuffer.c."

This was originally done in commit 5e77625b26 for 15 only, as a
troubleshooting aid but multiple people showed interest in back-patching
this.

Author: Jeremy Schneider
Reviewed-by: Amit Kapila
Backpatch-through: 9.6
Discussion: https://postgr.es/m/808ed65b-994c-915a-361c-577f088b837f@amazon.com

src/backend/replication/logical/reorderbuffer.c

@@ -4632,8 +4632,8 @@ ReorderBufferToastReplace(ReorderBuffer *rb, ReorderBufferTXN *txn,
 
 	toast_rel = RelationIdGetRelation(relation->rd_rel->reltoastrelid);
 	if (!RelationIsValid(toast_rel))
-		elog(ERROR, "could not open relation with OID %u",
-			 relation->rd_rel->reltoastrelid);
+		elog(ERROR, "could not open toast relation with OID %u (base relation \"%s\")",
+			 relation->rd_rel->reltoastrelid, RelationGetRelationName(relation));
 
 	toast_desc = RelationGetDescr(toast_rel);