Fix a number of places that produced XX000 errors in the regression tests.

It's against project policy to use elog() for user-facing errors, or to
omit an errcode() selection for errors that aren't supposed to be "can't
happen" cases.  Fix all the violations of this policy that result in
ERRCODE_INTERNAL_ERROR log entries during the standard regression tests,
as errors that can reliably be triggered from SQL surely should be
considered user-facing.

I also looked through all the files touched by this commit and fixed
other nearby problems of the same ilk.  I do not claim to have fixed
all violations of the policy, just the ones in these files.

In a few places I also changed existing ERRCODE choices that didn't
seem particularly appropriate; mainly replacing ERRCODE_SYNTAX_ERROR
by something more specific.

Back-patch to 9.5, but no further; changing ERRCODE assignments in
stable branches doesn't seem like a good idea.

src/backend/commands/vacuum.c

@@ -180,7 +180,10 @@ vacuum(int options, RangeVar *relation, Oid relid, VacuumParams *params,
 	 * calls a hostile index expression that itself calls ANALYZE.
 	 */
 	if (in_vacuum)
-		elog(ERROR, "%s cannot be executed from VACUUM or ANALYZE", stmttype);
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("%s cannot be executed from VACUUM or ANALYZE",
+						stmttype)));
 
 	/*
 	 * Send info about dead objects to the statistics collector, unless we are