fix: grey out next button in case of error

It should not be possible to click the Next button if there's
any error.

This could arise for a self-send if a very high feerate is chosen
and there are insufficient funds. In this case, the Next button
will now be greyed out until form values are edited and the error
has disappeared.

liana-gui/src/app/view/spend/mod.rs

@@ -300,6 +300,7 @@ pub fn create_spend_tx<'a>(
                     .push(
                         if is_valid
                             && !duplicate
+                            && error.is_none()
                             && (is_self_send || Some(&Amount::from_sat(0)) == amount_left)
                         {
                             button::secondary(None, "Next")