Nickserver should respond with a different repsonse code than 500 when querying an 1024 bit key

--- ~ » curl -k -i --silent --max-time 10 -X GET -d address=dsa1024@example.org https://unstable.bitmask.net:6425
HTTP/1.1 500 Internal Server Error
Date: Tue, 06 Dec 2016 14:50:46 GMT
Server: Apache
Content-Length: 103
X-Frame-Options: DENY
Connection: close

Ignoring key 9E54118C8C3066AAB874DDEA8545F671E87463F3 for dsa1024@example.org: key length is too short.%    

(from redmine: created on 2016-12-06, relates #8648 (closed))