g10: notify a user when importing stub is skipped.
* g10/import.c (transfer_secret_keys): Return GPG_ERR_NOT_PROCESSED when stub_key_skipped. (import_secret_one): Notify a user, suggesting --card-status.
Migration to 2.1 might be confusing with smartcard. With this patch,
a user can learn to run gpg ---card-status.
Thanks to intrigeri for the report.
- Debian-bug-id: #795881