bisect: fix exiting when checkout failed in bisect_start()

Commit 4796e823 ("bisect: introduce --no-checkout support into porcelain." Aug 4 2011)
made checking out the branch where we started depends on the "checkout" mode. But
unfortunately it lost the "|| exit" part after the checkout command.

As it makes no sense to continue if the checkout failed and as people have already
complained that the error message given when we just exit in this case is not clear, see:

http://thread.gmane.org/gmane.comp.version-control.git/180733/

this patch adds a "|| die <hopefully clear message>" part after the checkout command.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Jon Seymour <jon.seymour@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>