LibCore+Everywhere: Return ErrorOr from ConfigFile factory methods

I've attempted to handle the errors gracefully where it was clear how to
do so, and simple, but a lot of this was just adding
`release_value_but_fixme_should_propagate_errors()` in places.

Userland/Libraries/LibDesktop/AppFile.cpp

@@ -43,7 +43,7 @@ void AppFile::for_each(Function<void(NonnullRefPtr<AppFile>)> callback, StringVi
 }
 
 AppFile::AppFile(StringView path)
-    : m_config(Core::ConfigFile::open(path))
+    : m_config(Core::ConfigFile::open(path).release_value_but_fixme_should_propagate_errors())
     , m_valid(validate())
 {
 }