[securedrop-export] Log and/or return error message from exception

[sssoleileraaa]

Description

We just return ERROR_GENERIC here and nothing else, no logs. It would help admins, qa, and developers debug this issue to log.

https://github.com/freedomofpress/securedrop-export/blob/master/securedrop_export/entrypoint.py#L21-L25

[sssoleileraaa]

(for now just update code running on sd-export-usb in /opt/venvs/securedrop-export/lib/python3.7/site-packages/securedrop_export)

[gonzalo-bulnes]

@rocodes I think this is a good point, and it's relevant to the work you're doing in freedomofpress/securedrop-export#105 🙂

[gonzalo-bulnes]

Note: if we were to log the original error message, let's make sure we only log it to the debug level, so that regular log files don't contain distrusted strings.