Use elog, not Assert, to report failure to provide an outer snapshot.
As of commit
84f5c2908, 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