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.

A bit background: Without the change you get 'driver not supported' which sounds (at least to me) as if either docker or the kernel isn't supporting btrfs where in fact both are but the mountpoint wasn't btrfs.