Add error codes when vacuum discovers VM corruption

Commit fd6ec93bf8 and other previous work established the
principle that when an error is potentially reachable in case of on-disk
corruption but is not expected to be reached otherwise,
ERRCODE_DATA_CORRUPTED should be used. This allows log monitoring
software to search for evidence of corruption by filtering on the error
code.

Enhance the existing log messages emitted when the heap page is found to
be inconsistent with the VM by adding this error code.

Suggested-by: Andrey Borodin <x4mmm@yandex-team.ru>
Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/87DD95AA-274F-4F4F-BAD9-7738E5B1F905%40yandex-team.ru

src/backend/access/heap/vacuumlazy.c

@@ -2121,8 +2121,11 @@ lazy_scan_prune(LVRelState *vacrel,
 	else if (all_visible_according_to_vm && !PageIsAllVisible(page) &&
 			 visibilitymap_get_status(vacrel->rel, blkno, &vmbuffer) != 0)
 	{
-		elog(WARNING, "page is not marked all-visible but visibility map bit is set in relation \"%s\" page %u",
-			 vacrel->relname, blkno);
+		ereport(WARNING,
+				(errcode(ERRCODE_DATA_CORRUPTED),
+				 errmsg("page is not marked all-visible but visibility map bit is set in relation \"%s\" page %u",
+						vacrel->relname, blkno)));
+
 		visibilitymap_clear(vacrel->rel, blkno, vmbuffer,
 							VISIBILITYMAP_VALID_BITS);
 	}
@@ -2143,8 +2146,11 @@ lazy_scan_prune(LVRelState *vacrel,
 	 */
 	else if (presult.lpdead_items > 0 && PageIsAllVisible(page))
 	{
-		elog(WARNING, "page containing LP_DEAD items is marked as all-visible in relation \"%s\" page %u",
-			 vacrel->relname, blkno);
+		ereport(WARNING,
+				(errcode(ERRCODE_DATA_CORRUPTED),
+				 errmsg("page containing LP_DEAD items is marked as all-visible in relation \"%s\" page %u",
+						vacrel->relname, blkno)));
+
 		PageClearAllVisible(page);
 		MarkBufferDirty(buf);
 		visibilitymap_clear(vacrel->rel, blkno, vmbuffer,