core: Return CANCELED and BAD_PASSPHRASE error code on decryption.

* src/decrypt.c (op_data_t): Add field pkdecrypt_failed.
(_gpgme_decrypt_status_handler): Consult new field.
(parse_status_error): Handle some error codes.

The idea is to return only a limited set of error codes because a user
won't be able to understand the more esoteric codes.

• GnuPG-bug-id: T3270
• Signed-off-by: Werner Koch <wk@gnupg.org>