Return GPG_ERR_CARD_NOT_PRESENT when pinentry-mode=loopback.
Since there isn't a way to prompt the user to insert the smartcard when
pinentry-mode=loopback, return GPG_ERR_CARD_NOT_PRESENT instead of
GPG_ERR_NO_PIN_ENTRY.
- agent/divert-scd.c (ask_for_card): Return GPG_ERR_CARD_NOT_PRESENT
when pinentry-mode=loopback.