Radio menu items should have at least one item checked.

This is to force all platforms to match the same behavior in GTK+.

atom/browser/api/lib/menu-item.coffee

@@ -22,7 +22,7 @@ class MenuItem
     @checked = @checked ? false
     @submenu = @submenu ? null
 
-    throw new Error('Unknown menu type') if MenuItem.types.indexOf(@type) is -1
+    throw new Error("Unknown menu type #{@type}") if MenuItem.types.indexOf(@type) is -1
 
     @commandId = ++nextCommandId
     @click = =>