Improve some error messages with invalid indexes for REINDEX CONCURRENTLY

An invalid index is skipped when doing REINDEX CONCURRENTLY at table
level, with INDEX_CORRUPTED used as errcode.  This is confusing,
because an invalid index could exist after an interruption.  The errcode
is switched to OBJECT_NOT_IN_PREREQUISITE_STATE instead, as per a
suggestion from Andres Freund.

While on it, the error messages are reworded, and a hint is added,
telling how to rebuild an invalid index in this case.  This has been
suggested by Noah Misch.

Discussion: https://postgr.es/m/20231118230958.4fm3fhk4ypshxopa@awork3.anarazel.de

src/backend/commands/indexcmds.c

@@ -3526,10 +3526,11 @@ ReindexRelationConcurrently(const ReindexStmt *stmt, Oid relationOid, const Rein
 
 					if (!indexRelation->rd_index->indisvalid)
 						ereport(WARNING,
-								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-								 errmsg("cannot reindex invalid index \"%s.%s\" concurrently, skipping",
+								(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+								 errmsg("skipping reindex of invalid index \"%s.%s\"",
 										get_namespace_name(get_rel_namespace(cellOid)),
-										get_rel_name(cellOid))));
+										get_rel_name(cellOid)),
+								 errhint("Use DROP INDEX or REINDEX INDEX.")));
 					else if (indexRelation->rd_index->indisexclusion)
 						ereport(WARNING,
 								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
@@ -3578,10 +3579,11 @@ ReindexRelationConcurrently(const ReindexStmt *stmt, Oid relationOid, const Rein
 
 						if (!indexRelation->rd_index->indisvalid)
 							ereport(WARNING,
-									(errcode(ERRCODE_INDEX_CORRUPTED),
-									 errmsg("cannot reindex invalid index \"%s.%s\" concurrently, skipping",
+									(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+									 errmsg("skipping reindex of invalid index \"%s.%s\"",
 											get_namespace_name(get_rel_namespace(cellOid)),
-											get_rel_name(cellOid))));
+											get_rel_name(cellOid)),
+									 errhint("Use DROP INDEX or REINDEX INDEX.")));
 						else
 						{
 							ReindexIndexInfo *idx;

src/test/regress/expected/create_index.out

@@ -2581,7 +2581,8 @@ DROP INDEX concur_reindex_ind5_ccnew;
 DELETE FROM concur_reindex_tab4 WHERE c1 = 1;
 -- The invalid index is not processed when running REINDEX TABLE.
 REINDEX TABLE CONCURRENTLY concur_reindex_tab4;
-WARNING:  cannot reindex invalid index "public.concur_reindex_ind5" concurrently, skipping
+WARNING:  skipping reindex of invalid index "public.concur_reindex_ind5"
+HINT:  Use DROP INDEX or REINDEX INDEX.
 NOTICE:  table "concur_reindex_tab4" has no indexes that can be reindexed concurrently
 \d concur_reindex_tab4
         Table "public.concur_reindex_tab4"