Handshake errors are not shown correctly

[Blaisorblade]

I just had to upgrade the connection protocol. Only in the end I realized that, even though the server can throw a negotiation error with (error 'handin "unknown handin version: ~e" ver), it doesn't mean it's shown correctly:

handin-connect: bad protocol from server: "handin: unknown handin version: 'ver1.1"

That gets worse if you try to put a beginner-friendly message:

handin-connect: bad protocol from server: "handin: Achtung: Ihre Version des Abgabeplugins ist veraltet. Aus diesem Grund können wir Ihre Lösung nicht annehmen.\n\nUpgraden Sie bitte das Plugin umgehend und starten Sie DrRacket neu. Eine Anleitung finden Sie auch im Forum:\n\n  https://forum-...

This shouldn't be so bad for us because we have both autoupgrade and had warnings on submissions for a couple of days (and over a deadline), and I guess almost everybody got it. (0 mentions on the forum out of ~280 submitting students). Still, that's not beginner-appropriate.

In fairness, it's already cool a version number was exchanged, even though the client code was not tested for upgrades, and I should have probably tested this myself. I'll try to submit a PR at some point.