Return error when not checking out an entry due to dirtiness.

Return error when not checking out an entry due to dirtiness.

Without -f flag, 'git-checkout-index foo.c' issued an error message
when foo.c already existed in the working tree and did not match index.
However it did not return an error from the underlying checkout_entry()
function and resulted in a successful exit(0).

Signed-off-by: Junio C Hamano <junkio@cox.net>