On a Dedicated Server when a mission is loaded that requires mods that the server does not have or a player attempts to join a mission missing required mods there is currently no Message for missing mods on either of these events.
But more importantly there is no listing of required mods for mission selection causing the issues noted above, If the server was to show the required mods for that mission to any joining players and on the mission screen it would greatly help when selecting a mission.