[Segment Replication] Modify shrink exception to be more informative

server/src/internalClusterTest/java/org/opensearch/indices/replication/SegmentReplicationResizeRequestIT.java

@@ -87,7 +87,8 @@ public void testCreateShrinkIndexThrowsExceptionWhenReplicasBehind() throws Exce
                     .get()
             );
             assertEquals(
-                " For index [test] replica shards haven't caught up with primary, please retry after sometime.",
+                "Replication still in progress for index [test]. Please wait for replication to complete and retry. "
+                    + "Use the _cat/segment_replication/test api to check if the index is up to date (e.g. bytes_behind == 0).",
                 exception.getMessage()
             );
 

server/src/main/java/org/opensearch/action/admin/indices/shrink/TransportResizeAction.java

@@ -168,9 +168,11 @@ protected void clusterManagerOperation(
                                 .getSegments()
                                 .getReplicationStats().maxBytesBehind != 0) {
                                 throw new IllegalStateException(
-                                    " For index ["
+                                    "Replication still in progress for index ["
                                         + sourceIndex
-                                        + "] replica shards haven't caught up with primary, please retry after sometime."
+                                        + "]. Please wait for replication to complete and retry. Use the _cat/segment_replication/"
+                                        + sourceIndex
+                                        + " api to check if the index is up to date (e.g. bytes_behind == 0)."
                                 );
                             }