Fail with a clear error if 'git fat pull' is run on uninitialised repo.

This fixes issue #25 in the original project.
Moved requirement for init to `checkout` operation.

Since `checkout` op is where the repo really needs to be init'ed for git-fat
I moved the requirement to there. This way it is still triggered by a `pull`
op, but only at the end after any files have been synced.
Print fatal error message to stderr
Merge branch 'jmurty/ensure-init-before-pull' (PR #33)

* jmurty/ensure-init-before-pull:
  Print fatal error message to stderr
  Moved requirement for init to `checkout` operation.
  Fail with a clear error if 'git fat pull' is run on uninitialised repo.