Add an example of enrichment prior to sending to DLQ (#207)

Author: rockwotj

modules/configuration/pages/error_handling.adoc

@@ -172,7 +172,7 @@ output:
     - resource: processor_2 # Only errored messages, or delivery failures to processor_1, go here
 ----
 
-Finally, in cases where you want to route data differently depending on the type of error message, you can use a xref:components:outputs/switch.adoc[`switch` output]:
+If you want to route data differently based on the type of error message, you can use a xref:components:outputs/switch.adoc[`switch` output]:
 
 [source,yaml]
 ----
@@ -194,4 +194,18 @@ output:
           resource: processor_3
 ----
 
-// end::single-source[]
+Finally, you can attach additional metadata when routing messages to the dead-letter queue, such as the error message. This can be done by running a series of xref:components:processors/about.adoc[processors] before sending the data to the final xref:components:outputs/about.adoc[output].
+
+[source,yaml]
+----
+output:
+  fallback:
+    - reject_errored:
+        resource: processor_1 # Only non-errored messages go here
+    - processors:
+        - mutation: |
+            root.error = @fallback_error # Adds the error message before sending the message to the dead-letter queue output
+      resource: processor_2 # Only errored messages, or delivery failures to processor_1, go here
+----
+
+// end::single-source[]