Update mod-manager.lua

Fixes fileExists not working.

scripts/gui/mod-manager.lua

@@ -1,4 +1,3 @@
-
 local gui=require 'gui'
 local widgets=require 'gui.widgets'
 
@@ -10,6 +9,7 @@ function fileExists(filename)
 		return
 	else
 		file:close()
+		return true
 	end
 end
 function copyFile(from,to) --oh so primitive
@@ -303,4 +303,4 @@ if dfhack.gui.getCurFocus()~='title' then
     qerror("Can only be used in title screen")
 end
 local m=manager{}
-m:show()
+m:show()