fix Cannot free Volume XXX, because it is reserved by someone else

fix Cannot free Volume XXX, because it is reserved by someone else

- when something goes wrong, sometime, the volume_unused() is not called
  and the volume stay stuck
- always call volume_unused() in the "check_bail_out:"
- load_encryption_key() can return a new error: VOL_ENC_ERROR
  that will trigger the call to volume_unused()