LibCore+Everywhere: Return an Error from DirIterator::error()

This also removes DirIterator::error_string(), since the same strerror()
string will be included when you print the Error itself. Except in `ls`
which is still using fprintf() for now.

Userland/DevTools/HackStudio/Dialogs/ProjectTemplatesModel.cpp

@@ -111,7 +111,7 @@ void ProjectTemplatesModel::rescan_templates()
     // Iterate over template manifest INI files in the templates path
     Core::DirIterator di(ProjectTemplate::templates_path(), Core::DirIterator::SkipDots);
     if (di.has_error()) {
-        warnln("DirIterator: {}", di.error_string());
+        warnln("DirIterator: {}", di.error());
         return;
     }