zfs: improve error message, when dataset is not readable

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>
zfs: fix cases where lookupZfsDataset fails:

- don't abort if an unaccessible mount point is found
- check if mountpoint is really a zfs dataset

Signed-off-by: Jörg Thalheim <joerg@higgsboson.tk>