Re-enable custom stages

Author: tboeghk

solr/core/src/java/org/apache/solr/handler/component/SearchHandler.java

@@ -36,6 +36,7 @@
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
@@ -728,8 +729,7 @@ private static String stageInEnglish(int nextStage) {
       case STAGE_DONE:
         return "FINISHING";
       default:
-        throw new SolrException(
-            SolrException.ErrorCode.SERVER_ERROR, "Unrecognized stage:" + nextStage);
+        return String.format(Locale.ROOT, "CUSTOM (%s)", nextStage);
     }
   }