gpg: Fix error handling.
* g10/keygen.c (generate_subkeypair): Handle errors from pinentry.
Previously, when generating a subkey, gpg would ask for the passphrase
of the primary key. If that dialog is canceled, gpg would ask a
second time for a passphrase to protect the new subkey.
Fix this by handling the error.
- GnuPG-bug-id: T3212
- Signed-off-by: Justus Winter <email@example.com>