Change error code "already exists" to "non-fast forward"

The "already exists" error code relies on a change in git.git "next",
which is (as far as I can tell) in no released version of git.
On the other hand, the "non-fast forward" code has been supported
since git v1.7.10
Merge pull request #10 from fingolfin/older-git

Change error code "already exists" to "non-fast forward"