Fix several btrfs driver error messages

Several of the error messages were wrong, but not one of them was helpful, so this fixes them to include the important information (the actual error), and a correct description of which operation failed.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3893 from tianon/clean-up-btrfs-error-messages

Fix several btrfs driver error messages