cvsimport: die on cvsps errors

cvsimport: die on cvsps errors

We were not previously checking the exit status of cvsps at
all. If it exited before producing any useful output, we
ended up with an empty import, which caused a spew of
confusing error messages from other parts of git:

$ git-cvsimport foo
Initialized empty Git repository in ...
some error from cvsps
fatal: refs/heads/origin: not a valid SHA1
fatal: master: not a valid SHA1
warning: You appear to be on a branch yet to be born.
warning: Forcing checkout of HEAD.
fatal: just how do you expect me to merge 0 trees?
checkout failed: 256

Now we get:

$ git-cvsimport foo
Initialized empty Git repository in ...
some error from cvsps
git-cvsimport: fatal: cvsps reported error

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Martin Langhoff <martin@catalyst.net.nz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>