Do not let the user eliminate a recovery path if it's the only one

[nondiremanuel]

Currently with Liana it is not possible to create a descriptor without recovery paths. It can be then confusing for the user to have the possibility of deleting all the recovery paths in the custom setup flow.

Right now we manage the situation by not letting the user click on "Continue", but this is not optimal. We should prevent the user from going into a situation in which there is no recovery path in the first place since we don't support it.

This is currently applicable only to the custom setup since in the templates we don't allow the user adding or removing keys/paths.