Avoid unclear errors when jungfrau run has no trains to correct

If a Jungfrau run has no trains to correct, the notebook fail with unclear message related to the step_timer.

Description

• Fix StepTimer.timespan() if timings were still None.
• Use from logging import warn for no trains for a sequence displayed WARNING.
• Fail if there is no RAW sequence file to correct. (All sequence files are empty.)

How Has This Been Tested?

Tested with run /gpfs/exfel/exp/HED/202202/p003048/usr/Reports/r106/HED_IA1_JF500K1_correct_003048_r106_221104_081650.pdf

Relevant Documents (optional)

Types of changes

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

Checklist:

Reviewers

@schmidtp @kluyvert