Add a Kotlin function example that has a failure handler

inngest-test-server/src/main/kotlin/com/inngest/testserver/App.kt

@@ -26,6 +26,7 @@ fun Application.module() {
                 RestoreFromGlacier(),
                 ProcessUserSignup(),
                 TranscodeVideo(),
+                SlackFailureReport(),
             ),
         )
     }

inngest-test-server/src/main/kotlin/com/inngest/testserver/SlackFailureReport.kt

@@ -0,0 +1,36 @@
+package com.inngest.testserver
+
+import com.inngest.*
+
+class SlackFailureReport :
+    InngestFunction(),
+    WithFailureHandler {
+    override fun config(builder: InngestFunctionConfigBuilder): InngestFunctionConfigBuilder =
+        builder
+            .id("always-fail-fn")
+            .name("Always Fail Function")
+            .triggerEvent("always-fail-fn")
+
+    override fun execute(
+        ctx: FunctionContext,
+        step: Step,
+    ): String {
+        step.run("throw exception") {
+            throw RuntimeException("This function always fails")
+            "Step result"
+        }
+
+        return "Success"
+    }
+
+    override fun onFailure(
+        ctx: FunctionContext,
+        step: Step,
+    ): String {
+        step.run("send slack message") {
+            "Sending a message to Slack"
+        }
+
+        return "onFailure Success"
+    }
+}