IAM: Requesting access token for unknown subject (on remote server) yields error 500

[reinkrul]

Client responds:

{"detail":"server returned HTTP 500 (expected: 200)","status":500,"title":"RequestServiceAccessToken failed"}

Server logs:

level=warning msg="HandleTokenRequest failed" error="subject not found" module=Auth/iam operation=HandleTokenRequest operationID=HandleTokenRequest requestURI=/oauth2/d8e49bf4-9b89-42b5-bd5f-05b720eb1e74/token user="<nil>"

This should probably a status 400 with something more explanatory.