Ignore isnotexist errors for restrict paths

Handle the error when remounting certain files and paths as readonly if
they do not exist.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #24 from crosbymichael/is-not-exist-errors

Ignore isnotexist errors for restrict paths