Stop processing TransportDownsampleAction on failure (#94624) (#94628)

In #90593 we introduced a new failure path into
`TransportDownsampleAction` but omitted the `return` after completing
the listener so processing would continue (and fail further down,
completing the listener again). This commit adds the missing `return`.

Author: DaveCTurner

docs/changelog/94624.yaml

@@ -0,0 +1,5 @@
+pr: 94624
+summary: Stop processing `TransportDownsampleAction` on failure
+area: Rollup
+type: bug
+issues: []

x-pack/plugin/rollup/src/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleAction.java

@@ -163,6 +163,7 @@ protected void masterOperation(
                             "Rollup forbidden for index [" + sourceIndexName + "] with document level or field level security settings."
                         )
                     );
+                    return;
                 }
             }
         }