xds: include node ID in RPC failure status messages from the XdsClient

xds/src/main/java/io/grpc/xds/ClientXdsClient.java

@@ -2538,9 +2538,8 @@ void onError(Status error) {
         respTimer = null;
       }
 
-      // Include node ID in RPC failure status messages that originate from XdsClient.
-      // We expect all watchers to propagate the description to the channel, and expose it
-      // to the caller.
+      // Include node ID in xds failures to allow cross-referencing with control plane logs
+      // when debugging.
       String description = error.getDescription() == null ? "" : error.getDescription() + " ";
       Status errorAugmented = Status.fromCode(error.getCode())
           .withDescription(description + "nodeID: " + bootstrapInfo.node().getId())

xds/src/main/java/io/grpc/xds/XdsClient.java

@@ -398,6 +398,12 @@ interface ResourceWatcher {
 
     /**
      * Called when the resource discovery RPC encounters some transient error.
+     *
+     * <p>Note that we expect that the implementer to:
+     * - Override {@link Status} code with UNAVAILABLE to comply with the guarantee to not generate
+     *   certain statuses by the library: https://grpc.github.io/grpc/core/md_doc_statuscodes.html
+     * - Keep {@link Status} description in one form or another, as it contains valuable debugging
+     *   information, such as node id.
      */
     void onError(Status error);