Improve error message if the dependent world is changing and incopatible with previous attemp at importing

[mikkelhegn]

As I try and get my World etc. right, I'm continuously trying to import it to a Spin component using spin deps add.

At a given point I ended up with this error:

Error: failed to merge package `root:component` into existing copy

Caused by:
    0: failed to merge world `dependency-world`
    1: import `advent-of-spin:challenge-two/non` not found in target world

It would be good to expand the error message telling the user where the root:component world can be found - .wit folder, and even better a way to just undo as a command.