update get_mount_point() to return EngineResult<Option<PathBuf>>
update get_mount_point() to return EngineResult<Option<PathBuf>>

The change allows the caller to distinguish between the filesytem being
unmounted and an error determining if it is mounted.

Signed-off-by: Todd Gill <7afbb5ba98932b8ffb9cc9e6675c4792db142529@redhat.com>

stratis-storage/stratisd,mulkieran/stratisd,stratis-storage/stratisd,trgill/stratisd,stratis-storage/stratisd,mulkieran/stratisd,trgill/stratisd