Failure to submit sysvars should return proper HTTP status code

[TinkyWonky]

Situation
Trying to submit new values for existing system variables using statechange.cgi, the provided parameter string is ill-formatted, like ?ise_foo=42&new_bar=baz.

Current Behaviour
The returned result code is 200 ("OK").

Suggested Behaviour
HTTP status code indicate that something went wrong. Perhaps in detail:

• Ill-formatted ?ise_id=... and &new_value=... result in code 400 ("Bad Request").
• Invalid ide_id values (system variable not known) result in code 422 ("Unprocessable Entity "). Maybe 406 ("Not Acceptable") or 416 ("Requested range not satisfiable")