Fill in nice error message for self-checkin-attempts

[RakanHowls]

When the API is updated to return a dedicated error code to show that the user is trying to do a self-check-in, a nice error message needs to be added

[dogpixels]

Just out of curiosity, is this a security measure to prevent unauthorized check-ins?

[RakanHowls]

@dogpixels I don't know :) I was surprised and confused by the error message from the API myself when I tried it as a final test on the live system... :)

Unfortunately, with the available data from the IDP and RegSys APIs, I can't know with 100% certainty if someone is trying to check himself in (Nicknames can be different in the IDP vs. RegSys, and the RegSys doesn't provide IDP IDs), so I need a new error code / error details from the backend to properly display why the checkin fails in that case