Merge: Fix nitpick return value

Make that nitpick returns 1 if some errors where detected.
This is more POLA and will helps callers to spot if there was errors in nit files given in arguments.

Pull-Request: #1581
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>