A couple buildmode fixes

The runtime when not placing a valid path is now replaced with a warning
message

The HUD icon for buildmode is no longer covered up by item actions

code/_onclick/hud/_defines.dm

@@ -161,3 +161,8 @@
 //1 = standard hud
 //2 = reduced hud (just hands and intent switcher)
 //3 = no hud (for screenshots)
+
+
+#define HUD_LAYER_SCREEN 20
+
+#define HUD_LAYER_BUILDMODE 30

code/_onclick/hud/screen_objects.dm

@@ -9,7 +9,7 @@
 /obj/screen
 	name = ""
 	icon = 'icons/mob/screen_gen.dmi'
-	layer = 20
+	layer = HUD_LAYER_SCREEN
 	plane = HUD_PLANE
 	unacidable = 1
 	var/obj/master = null	//A reference to the object in the slot. Grabs or items, generally.