Harden legacy config file handling

Use the native io.exists helper and return nil when config reads fail instead of raising fatal errors, allowing callers to handle missing or unreadable configuration files gracefully.

YimMenu/Ultimate_Menu_Legacy.lua

@@ -4528,13 +4528,7 @@ function YimConfig(DEFAULT_CONFIG)
     }
 
     function file_exists(name)
-        local f = io.open(name, "r")
-        if f ~= nil then
-            f:close()
-            return true
-        else
-            return false
-        end
+        return io.exists(name)
     end
 
     local writeToFile = function(data)
@@ -4575,7 +4569,7 @@ function YimConfig(DEFAULT_CONFIG)
         else
             config = readFromFile()
             if config == nil then
-                error("[FATAL] Failed to read config file!")
+                return nil
             end
 
             for key, defaultValue in pairs(DEFAULT_CONFIG) do
@@ -4589,7 +4583,7 @@ function YimConfig(DEFAULT_CONFIG)
 
     local readAndDecodeConfig = function()
         if not checkAndCreateConfig() then
-            error("Failed to read config file!", 2)
+            return nil
         end
         return readFromFile()
     end