agent: Improve error reporting from Pinentry.
* agent/call-pinentry.c (unlock_pinentry): Add error logging. Map error source of uncommon errors to Pinentry.
With this change it is possible to detect whether an error like
GPG_ERR_ASS_INV_RESPONSE has its origin in a call to Pinentry or comes
from another part of gpg-agent.
- Signed-off-by: Werner Koch <wk@gnupg.org>