resume pulling the layer on disconnect

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
switch skipping from error to debug

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #6166 from vieux/debug_skipping

switch skipping from error to debug
Merge pull request #5848 from unclejack/resumable_pulls

resume pulling the layer on disconnect