Fail when import of signatures code fails

This MR concerns only new JO menu integration. It changes behaviour in case of an error in signatures code. Now the issue is propagated upstream. Before there only a warning printed + the stack trace was missing. It became impossible to do any debugging with it. Another modified aspect was that if certain signature is not configured (i.e. list of chains is == []) respective signature code is not imported at all. I think @fpastore was asking for this fix long time ago already. Tagging @kjamrog