Add alert for failed events after retry and fix broken cloud event exception handling

Relates #3 (closed)

example/main.py

@@ -19,6 +19,15 @@ def example_raw_event(raw_event):
     patch_me(raw_event)
 
 
+@ucam_faas.raw_event
+def example_raw_event_fail(raw_event):
+    logger.info("got_raw_event", raw_event=raw_event)
+
+    patch_me(raw_event)
+
+    raise ucam_faas.exceptions.UCAMFAASCouldNotProcess
+
+
 @ucam_faas.cloud_event
 def example_cloud_event(cloud_event):
     logger.info("got_cloud_event", cloud_event=cloud_event)