Add missing SPAWN_FAILED message key

Author: benwoo1110

src/main/resources/multiverse-core_en.properties

@@ -135,6 +135,7 @@ mv-core.setspawn.world.description=Target world to set spawn of (defaults to pla
 mv-core.spawn.description=Teleports the specified player to the spawn of the world they are in
 mv-core.spawn.player.description=The player
 mv-core.spawn.success={teleporter} just sent you to spawn!
+mv-core.spawn.failed={teleporter} failed to teleport you to spawn :(
 mv-core.spawn.consolename=The console
 mv-core.spawn.you=You