TSV missing value handling fixes and enhancement
TSV missing value handling fixes and enhancement

Change error code from non-existent 23 to 24 (which describes the
error).

Potentially Fixes #66

suyashdb/bids-validator,Squishymedia/BIDS-Validator,nellh/bids-validator,nellh/bids-validator,nellh/bids-validator,chrisfilo/bids-validator,Squishymedia/bids-validator