Show error message if checkbox wasn’t checked

Because we were redirecting in all cases the error message wasn’t being
shown.

This commit changes the endpoint to respond with content (including an
error message) if the `POST` is not successful.

app/main/forms.py

@@ -1297,7 +1297,9 @@ class ConfirmBroadcastForm(StripWhitespaceForm):
         self.confirm.label.text = self.generate_label(channel, max_phones)
 
         if service_is_live:
-            self.confirm.validators += (DataRequired(),)
+            self.confirm.validators += (
+                DataRequired('You need to confirm that you understand'),
+            )
 
     confirm = GovukCheckboxField("Confirm")