Use track metadata from Airtime in playout engine

* Resolves CC-5893: Metadata not updated on Airtime Pro
* Report track metadata in the schedule API, and make pypo pass that
  along to Liquidsoap via annotations.
* Move HTTP response sanitization for file metadata out of the REST module and into CcFiles
* Slightly improved the terrible exception handling in pypo

python_apps/pypo/liquidsoap_scripts/generate_liquidsoap_cfg.py

@@ -44,5 +44,8 @@ while not successful:
             logging.error("traceback: %s", traceback.format_exc())
             sys.exit(1)
         else:
+            logging.error(str(e))
+            logging.error("traceback: %s", traceback.format_exc())
+            logging.info("Retrying in 3 seconds...")
             time.sleep(3)
     attempts += 1