Missing HDF5 layout check when loading list of files into GUI
Steps to reproduce:
- Find a multi-frame-per-file dataset, e.g. Eiger data in NeXus format
- Create a list of files (without event IDs)
- Open GUI with an empty session
- Load data, select list of files, give it the list from step 2, plus the correct geometry file
- Observe that one frame of data is shown, but event ID shows as "//".
- Click Next Image -> HDF5 error messages in terminal
It shouldn't be possible to get any image visible at all in the GUI in step 5. It should fail (preferably with a useful error message) before then.