sql: return an error in writeZoneConfig if non-system tenant

This code path was called directly from schema change code, resulting in errors
like "relation system.zones does not exist". This commit makes the reason for
the failure clearer (tenants cannot set zone configs).

Release note: None (multitenancy work)

pkg/sql/set_zone_config.go

@@ -103,7 +103,7 @@ func (p *planner) SetZoneConfig(ctx context.Context, n *tree.SetZoneConfig) (pla
 		return nil, err
 	}
 	if !p.ExecCfg().Codec.ForSystemTenant() {
-		return nil, errorutil.UnsupportedWithMultiTenancy()
+		return nil, errorutil.UnsupportedWithMultiTenancy(multitenancyZoneCfgIssueNo)
 	}
 
 	var yamlConfig tree.TypedExpr
@@ -837,6 +837,8 @@ func validateZoneAttrsAndLocalities(
 	return nil
 }
 
+const multitenancyZoneCfgIssueNo = 49854
+
 func writeZoneConfig(
 	ctx context.Context,
 	txn *kv.Txn,
@@ -846,6 +848,9 @@ func writeZoneConfig(
 	execCfg *ExecutorConfig,
 	hasNewSubzones bool,
 ) (numAffected int, err error) {
+	if !execCfg.Codec.ForSystemTenant() {
+		return 0, errorutil.UnsupportedWithMultiTenancy(multitenancyZoneCfgIssueNo)
+	}
 	if len(zone.Subzones) > 0 {
 		st := execCfg.Settings
 		zone.SubzoneSpans, err = GenerateSubzoneSpans(