Errors returned by unix are bare. In some cases it's impossible to find
out what went wrong because there's is not enough context.
Add a mountError type (mostly copy-pasted from github.com/moby/sys/mount),
and mount/unmount helpers. Use these where appropriate, and convert error
checks to use errors.Is.
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>