core: Return bad data error instead of general error on unexpected data

* src/verify.c (_gpgme_verify_status_handler): Return GPG_ERR_BAD_DATA
instead of GPG_ERR_GENERAL.
--

This gives users trying to decrypt and/or verify for example a public
key block a more useful error.

GnuPG-bug-id: 6617

src/verify.c

@@ -979,7 +979,7 @@ _gpgme_verify_status_handler (void *priv, gpgme_status_code_t code, char *args)
     case GPGME_STATUS_UNEXPECTED:
       opd->only_newsig_seen = 0;
       if (!sig)
-	return gpg_error (GPG_ERR_GENERAL);
+	return gpg_error (GPG_ERR_BAD_DATA);
       sig->status = gpg_error (GPG_ERR_NO_DATA);
       break;