erp5_wendelin: Improve error handling in ingestion script

Improve robustness of the default ingestion script: similar to the IngestionPolicy component that calls this script, throw a NotFound exception if either a Data Ingestion Line or a Data Operation is not found, instead of crashing the process.

In fluentd logs, this transforms a 500 status code response into a 404 status code, indicating that something was indeed not correctly set up.

I use this modification in ORS Wendelin for easier debugging and clarification, I think it could be useful for the main branch as well.