Be more explicit about tarball: NewImageDestination failing
Be more explicit about tarball: NewImageDestination failing

Don't imply that we're planning on adding writing to the tarball:
transport.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

containers/image,containers/image