[BUG] Index.lock error message regression in git 2.11.0

Hello,

I just upgraded to 2.11.0 from 2.10.2, and I noticed that some
commands no longer print an error message when the `index.lock` file
exists (such as `git merge --ff-only`).

It appears this bug was introduced in
55f5704da69d3e6836620f01bee0093ad5e331e8 (sequencer: lib'ify
checkout_fast_forward()). I determined this by running the attached
bisect script (on OS X, but I don't think that matters; I've also seen
the error message missing on Linux):

$ cd /path/to/git/src
$ git bisect start v2.11.0-rc0 v2.10.2
$ git bisect run /path/to/bisect.test.sh

(my original version of the script also includes some other makefile
parameters to get a modern version of gettext and openssl too, but
they're not relevant to this ML).

I'm not certain that I have enough context to propose a meaningful
patch though :/.

Cheers,
Robbie