Add effect failure

The apply function of effects can return false, in that case, the effect
is not applied

examples.lua

@@ -1,4 +1,13 @@
 ----- EXAMPLE EFFECT TYPES -----
+--[[ Null effect. The apply function always returns false, which means applying the
+effect will never succeed ]]
+playereffects.register_effect_type("null", "No effect", nil, {},
+	function()
+		return false
+	end
+)
+
+
 -- Makes the player screen black for 5 seconds (very experimental!)
 playereffects.register_effect_type("blind", "Blind", nil, {},
 	function(player)
@@ -8,7 +17,12 @@ playereffects.register_effect_type("blind", "Blind", nil, {},
 			scale = { x=-100, y=-100 },
 			text = "playereffects_example_black.png",
 		})
-		return { hudid = hudid }
+		if(hudid ~= nil) then
+			return { hudid = hudid }
+		else
+			minetest.log("error", "[playereffects] [examples] The effect \"Blind\" could not be applied. The call to hud_add(...) failed.")
+			return false
+		end
 	end,
 	function(effect)
 		local player = minetest.get_player_by_name(effect.playername)
@@ -91,6 +105,16 @@ playereffects.register_effect_type("stress", "Stress Test Effect", nil, {},
 
 
 ------ Chat commands for the example effects ------
+-- Null effect (never succeeds)
+minetest.register_chatcommand("null", {
+	params = "",
+	description = "Does nothing.",
+	privs = {},
+	func = function(name, param)
+		playereffects.apply_effect_type("null", 5, minetest.get_player_by_name(name))
+	end,
+})
+
 minetest.register_chatcommand("blind", {
 	params = "",
 	description = "Makes your screen black for a short time.",

init.lua

@@ -69,7 +69,17 @@ end
 
 function playereffects.apply_effect_type(effect_type_id, duration, player)
 	local start_time = os.time()
+	local status = playereffects.effect_types[effect_type_id].apply(player)
 	local playername = player:get_player_name()
+	local metadata
+
+	if(status == false) then
+		minetest.log("action", "[playereffects] Attempt to apply effect type "..effect_type_id.." to player "..playername.." failed!")
+		return
+	else
+		metadata = status
+	end
+
 	local groups = playereffects.effect_types[effect_type_id].groups
 	for k,v in pairs(groups) do
 		playereffects.cancel_effect_group(v, playername)
@@ -105,8 +115,6 @@ function playereffects.apply_effect_type(effect_type_id, duration, player)
 		hudids = {text_id=nil, icon_id=nil}
 	end
 
-	local metadata = playereffects.effect_types[effect_type_id].apply(player)
-
 	local effect = {
 			playername = playername, 
 			effect_id = effect_id,