Command line option to have barcheck failures cause errors instead of warnings

[Urs Liska]

Hi,

there was the request for a way to have barcheck failures cause errors
instead of warnings:
http://lists.gnu.org/archive/html/lilypond-user/2015-12/msg00074.html

I think this is a valid use case as very often you don't actually want
to have the compiled score when it contains barcheck failures, erroring
out with a useful message pointing to the input location can be a
time-saver.

This could be controlled by a command line option (e.g.
-derror-on-barcheck).

Before giving it a try and preparing a patch I would like to get some
feedback if this idea seems good.

Best
Urs