Python library: Add some error code for volume masking
Python library: Add some error code for volume masking

To simplify LSM user and plugin implementation, I would like to restrict
these:
    1. access_group_initiator_delete()
       Refuse to delete the last initiator from access group which has
       volume masked.
       Some plugin(SMI-S for EMC VMAX) does not allow keeping volume masking
       information when no member in access group.

    2. volume_mask()
       Refuse to do volume masking against empty(no member) access group.
       The same reason as previous one.

    3. access_group_delete():
       Refuse to delete a access group which has volume masked.

Signed-off-by: Gris Ge <e24f3acbe39bf94f92e8af3c48f8852106ad5ad7@redhat.com>

cathay4t/libstoragemgmt,HP-Scale-out-Storage/libstoragemgmt,cathay4t/libstoragemgmt,cathay4t/libstoragemgmt,HP-Scale-out-Storage/libstoragemgmt,jumitche/libstoragemgmt,libstorage/libstoragemgmt,cathay4t/libstoragemgmt,jumitche/libstoragemgmt,libstorage/libstoragemgmt,libstorage/libstoragemgmt,joehandzik/libstoragemgmt,cathay4t/libstoragemgmt,jumitche/libstoragemgmt,HP-Scale-out-Storage/libstoragemgmt,tasleson/libstoragemgmt,lazy404/libstoragemgmt,tasleson/libstoragemgmt,libstorage/libstoragemgmt,tasleson/libstoragemgmt,joehandzik/libstoragemgmt,lazy404/libstoragemgmt,lazy404/libstoragemgmt,tasleson/libstoragemgmt,tasleson/libstoragemgmt,joehandzik/libstoragemgmt,jumitche/libstoragemgmt,lazy404/libstoragemgmt,jumitche/libstoragemgmt,joehandzik/libstoragemgmt,HP-Scale-out-Storage/libstoragemgmt,libstorage/libstoragemgmt