No error when channels.tsv files are not placed in the `meg` folder

[Remi-Gau]

trafficstars

See issue in the bids example: https://github.com/bids-standard/bids-examples/issues/296

This is probably a bug.

Suggested course of action.

• open a PR to fix validator to throw an error on that dataset
• open a PR to fix dataset in the examples
• merge everything
• and they lived happily everafter