libct/rootfs: improve some errors
libct/rootfs: improve some errors

Errors from os.Open, os.Symlink etc do not need to be wrapped, as they
are already wrapped into os.PathError.

Error from unix are bare errnos and need to be wrapped. Same
os.PathError is a good candidate.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

rhatdan/runc,opencontainers/runc,dqminh/runc,dqminh/runc,opencontainers/runc,dqminh/runc,dqminh/runc,opencontainers/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,kolyshkin/runc,rhatdan/runc,kolyshkin/runc,opencontainers/runc,rhatdan/runc