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/file_system/program.dm

@@ -179,8 +179,8 @@
 	ui = SStgui.try_update_ui(user, src, ui)
 	if(!ui && tgui_id)
 		ui = new(user, src, tgui_id, filedesc)
-		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))
 
 // CONVENTIONS, READ THIS WHEN CREATING NEW PROGRAM AND OVERRIDING THIS PROC:
 // Topic calls are automagically forwarded from NanoModule this program contains.