Fixes #123. The code assumes that every File that has its link_target() method called would first have been checked to make sure it’s actually a link first. Unfortunately it also assumed that the only thing that can go wrong while following a link is if the file wasn’t a link, meaning it crashes when given a link it doesn’t have permission to follow.

This makes the file_target() method able to return either a file or path for displaying, as before, but also an IO error for when things go wrong.