bug#24083: guix lint doesn't actually fail on some errors

[Danny Milosavljevic]

guix lint doesn't actually fail on some errors.

$ guix lint perl-io-socket-inet6 ; echo $?
;;; note: source file /x/home/dannym/src/guix/gnu/packages/networking.scm
;;;       newer than compiled /x/home/dannym/src/guix/gnu/packages/networking.go
;;; note: source file /x/home/dannym/src/guix/gnu/packages/mail.scm
;;;       newer than compiled /x/home/dannym/src/guix/gnu/packages/mail.go
;;; note: source file /x/home/dannym/src/guix/gnu/packages/ldc.scm
;;;       newer than compiled /x/home/dannym/src/guix/gnu/packages/ldc.go
gnu/packages/networking.scm:437:1: perl-io-socket-inet6-2.72: line 464 is way 
too long (96 characters)
gnu/packages/networking.scm:437:1: perl-io-socket-inet6-2.72: line 483 is way 
too long (92 characters)
0
^--- ooops