Fix runtime with NTOS apps and improve code involved (#53779)

There exists a specific state where ui.open() can be called yet the 
result would be a null window and such behaviour would be intentional. 
The following CRASH in ui.send_asset() would thus be misleading, 
because send_asset() was called after open().

This PR adds more information to the CRASH about when the failure state 
can occur, makes open() return a value based on whether it actually 
opened a new pooled window or not, and makes sure modular computer apps 
don't send_assets unless a new pooled window was created.

code/modules/modular_computers/computers/item/computer_ui.dm

@@ -37,8 +37,8 @@
 	if (!ui)
 		ui = new(user, src, "NtosMain")
 		ui.set_autoupdate(TRUE)
-		ui.open()
-		ui.send_asset(get_asset_datum(/datum/asset/simple/headers))
+		if(ui.open())
+			ui.send_asset(get_asset_datum(/datum/asset/simple/headers))
 
 
 /obj/item/modular_computer/ui_data(mob/user)