[JUNGFRAU][CORRECT] Warn and Exit if all files are empty

Description

As FXE has multiple occurrences with empty sequences in Jungfrau runs. It is unfortunately expected to have slurm jobs with no sequence files of valid data to correct.

• Replace the error with a warning.
• Exit instead of continuing with the plotting cells.

How Has This Been Tested?

Relevant Documents (optional)

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

Reviewers

@schmidtp @danilevc