Make 'headerscheck' stop immediately on an error

This should make it stop immediately after printing the _helpful_ error
message, rather than continuing to spit out many pages more of 'CHECK
include/linux/foo.h' before eventually coming to a halt with something
less obvious.

Now I get this...
  CHECK   include/linux/smb_fs.h
/shiny/git/linux-2.6/usr/include/linux/smb_fs.h requires linux/jiffies.h, which does not exist in exported headers
make[2]: *** [/shiny/git/linux-2.6/usr/include/linux/.check.smb_fs.h] Error 1
make[1]: *** [linux] Error 2
make: *** [headers_check] Error 2

Signed-off-by-if-Sam-says-so: David Woodhouse <dwmw2@infradead.org>
[ Sam had better say so! This made me waste way too much time. - Linus]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>