Invisible Wall collide tweak (#4247)

# About the pull request
Changes where the message comes from when collidin with an invisible
wall as it was hardcoded but using the same text as description var
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# Explain why it's good for the game
# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog
:cl:
code: Changed backend for the message of walking into an invisible wall
to reduce duplication.
/:cl:

code/game/objects/structures/blocker.dm

@@ -25,7 +25,10 @@
 	mouse_opacity = MOUSE_OPACITY_TRANSPARENT
 
 /obj/structure/blocker/invisible_wall/Collided(atom/movable/AM)
-	to_chat(AM, SPAN_WARNING("You cannot go this way."))
+	var/msg = desc
+	if(!msg)
+		msg = "You cannot go this way."
+	to_chat(AM, SPAN_WARNING(msg))
 
 /obj/structure/blocker/invisible_wall/New()
 	..()
@@ -35,10 +38,6 @@
 	desc = "You cannot wade out any further"
 	icon_state = "map_blocker"
 
-/obj/structure/blocker/invisible_wall/water/Collided(atom/movable/AM)
-	to_chat(AM, SPAN_WARNING("You cannot wade out any further."))
-
-
 /obj/structure/blocker/fog
 	name = "dense fog"
 	desc = "It looks way too dangerous to traverse. Best wait until it has cleared up."