LibCore+Everywhere: Return ErrorOr from ConfigFile::sync()

Currently this method always succeeds, but that won't be true once we
switch to the Core::Stream API. :^)

Some of these places would ideally show an error message to the user,
since failure to save a file is significant, but let's not get
distracted right now.

Userland/Libraries/LibCore/ConfigFile.h

@@ -52,7 +52,7 @@ public:
 
     bool is_dirty() const { return m_dirty; }
 
-    bool sync();
+    ErrorOr<void> sync();
 
     void remove_group(String const& group);
     void remove_entry(String const& group, String const& key);