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/LibGfx/SystemTheme.cpp

@@ -147,7 +147,7 @@ Core::AnonymousBuffer load_system_theme(Core::ConfigFile const& file)
 
 Core::AnonymousBuffer load_system_theme(String const& path)
 {
-    return load_system_theme(Core::ConfigFile::open(path));
+    return load_system_theme(Core::ConfigFile::open(path).release_value_but_fixme_should_propagate_errors());
 }
 
 }