Fix image pull error handling

This improves error handling for non-existing images:

  [penberg@localhost capstan]$ ./install && capstan pull foo
  Pulling foo...
  foo: no such remote image

and when image is already pulled to local repository:

  [penberg@localhost capstan]$ ./install && capstan pull penberg/osv-base
  penberg/osv-base: image already exists

Please note that eventually we need to use "git pull" for the latter to
update the image.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>