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 <justus@g10code.com>