Display which projects were cleaned up

Hello,

This improves `projectile-cleanup-known-projects`.

I added an additional `ignore-errors` in there, so it'd not stop on old dead TRAMP invalid hosts and continue cleaning up. I'm not completely sure about this, because it means that if you refuse to log in your host it'll remove the remote projects... but at the same time you're supposed to log in in order for projectile to check the path is still valid.

@thomasf: what's your take on the `ignore-errors` part?
You can simplify this to `-separate 'projectile-keep-project-p`