`emigrate`: allowing dwarfs who can't leave to emigrate causes issues

[ab9rf]

per discussion on kitfox:

a dwarf who was unable to path off the map was emigrated by emigration causing him to go insane when he could not path off the map, causing caravans to get stuck

recommend solution: only allow dwarfs to emigrate if they can path to the trade depot that the caravan they're going to leave with is at