pop_importbids error not informative enough when loading non-BIDS compliant data

[claire-braboszcz]

Trying to load a BIDS dataset that is not fully BIDS-compliant (eg. missing participants.tsv file) with "Import BIDS folder to study" leads to error message "no dataset found" that could be made better to help understand where the issue comes from:

https://github.com/sccn/EEG-BIDS/blob/f6d5ef29dbd2eca215fff8359862d05fe30c668b/pop_importbids.m#L749