Parse path name lines live and festoon callbacks with error handling

[adamnovak]

This should fix #472 in two ways.

We no longer make one giant string of all the path names (though we do call res.json() with one giant object), and we now have some error handlers in the callbacks in that call (but not others) that catch errors and call next().

[adamnovak]

I've also synced up subpath handling with vgteam/vg#4522 so that we can do the coordinate bars in terms of base paths, and handle queries in either base paths or single subpaths.