Use elog, not Assert, to report failure to provide an outer snapshot.

As of commit 84f5c290, executing SQL commands (via SPI or otherwise)
requires having either an active Portal, or a caller-established
active snapshot.  We were simply Assert'ing that that's the case.
But we've now had a couple different reports of people testing
extensions that didn't meet this requirement, and were confused by
the resulting crash.  Let's convert the Assert to a test-and-elog,
in hopes of making the issue clearer for extension authors.

Per gripes from Liu Huailing and RekGRpth.  Back-patch to v11,
like the prior commit.

Discussion: https://postgr.es/m/OSZPR01MB6215671E3C5956A034A080DFBEEC9@OSZPR01MB6215.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/17035-14607d308ac8643c@postgresql.org

src/backend/tcop/pquery.c

@@ -1758,7 +1758,8 @@ EnsurePortalSnapshotExists(void)
 
 	/* Otherwise, we'd better have an active Portal */
 	portal = ActivePortal;
-	Assert(portal != NULL);
+	if (unlikely(portal == NULL))
+		elog(ERROR, "cannot execute SQL without an outer snapshot or portal");
 	Assert(portal->portalSnapshot == NULL);
 
 	/* Create a new snapshot and make it active */