new Launcher scripts doing insufficient error checking with subprocess

See the output in the linked issue #3523 (closed)...

Preparation set as finished.
>> Scanning of papers
Downloading classlist to '/tmp/tmpo071b9da/classlist.csv'
	Prenaming is enabled, so saving 'paper_number' column
CommandError: Unknown command: 'plom_preparation_extrapage'
Uploaded fake_bundle1.pdf as user demoScanner1 - processing it in the background now.
Uploaded fake_bundle2.pdf as user demoScanner1 - processing it in the background now.
Uploaded fake_bundle3.pdf as user demoScanner1 - processing it in the background now.

Something is raising CommandError but the code is not stopping. I suspect no error checking being done in subprocess, b/c IIRC the launcher scripts use subprocess.

(Also, there is the matter of that command being missing, but that's probably an easy fix and not the point of this bug.)