Replace deprecated entry in launchd config with the modern equivalent.
Merge pull request #854 from SaberUK/insp20+deprecated-plist-entry

Replace deprecated entry in launchd config with the modern equivalent.
Update server.cpp

Rewrote "invalid credentials" error message to remove confusion regarding "server snomask", which some users interpreted as "I must make sure the snomask on each server is identical" (which makes no sense), when really the intention is basically "look at the logs on the other server".