* do not consider iptables' output an error in case of xtables lock
Docker-DCO-1.1-Signed-off-by: Giuseppe Mazzotta <gdm85@users.noreply.github.com> (github: gdm85)
Add ErrPrerequisites to improve misleading errors

There are two cases where we can't use a graphdriver:

1) the graphdriver itself isn't supported by the system
2) the graphdriver is supported by some configuration/prerequisites are
missing

This introduces a new error for the 2) case and uses it when trying to
run docker with btrfs backend on a non-btrfs filesystem.

Docker-DCO-1.1-Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org> (github: discordianfish)
Merge pull request #6106 from discordianfish/improve-error-btrfs-on-non-btrfs

Add ErrPrerequisites to improve misleading errors
Merge pull request #6105 from gdm85/master

Do not consider iptables' output an error in case of xtables lock