Improve REPACK (CONCURRENTLY) error when wal_level < replica

The error emitted when REPACK (CONCURRENTLY) is run with too low a
wal_level is thrown by CheckSlotRequirements(), which is a bit
mysterious when the user doesn't know what's up.  Add an upfront check
in check_concurrent_repack_requirements() for a more explicit, REPACK-
centered report, which is easier to understand -- this also saves
starting the worker just to have it die immediately.

Author: Baji Shaik <baji.pgdev@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/CA+fm-ROdgh0rEVuXoViBk4TVgjodrN=MTR_RYuOuKLZ9voX4YA@mail.gmail.com

src/backend/commands/repack.c

@@ -40,6 +40,7 @@
 #include "access/toast_internals.h"
 #include "access/transam.h"
 #include "access/xact.h"
+#include "access/xlog.h"
 #include "catalog/catalog.h"
 #include "catalog/dependency.h"
 #include "catalog/heap.h"
@@ -897,6 +898,14 @@ check_concurrent_repack_requirements(Relation rel, Oid *ident_idx_p)
 				replident;
 	Oid			ident_idx;
 
+	if (wal_level < WAL_LEVEL_REPLICA)
+		ereport(ERROR,
+				errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				errmsg("cannot execute \"%s\" in this configuration",
+					   "REPACK (CONCURRENTLY)"),
+				errdetail("%s requires \"wal_level\" to be set to \"replica\" or higher.",
+						  "REPACK (CONCURRENTLY)"));
+
 	/* Data changes in system relations are not logically decoded. */
 	if (IsCatalogRelation(rel))
 		ereport(ERROR,