DEV: Fix autoloading (#322)

A follow-up to e23c31195b

Fixes errors like:
```
NameError:
       uninitialized constant DiscourseDataExplorer::ValidationError
     # ./plugins/discourse-data-explorer/spec/lib/parameter_spec.rb:15:in `block (3 levels) in <main>'
```

lib/discourse_data_explorer/validation_error.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+module ::DiscourseDataExplorer
+  class ValidationError < StandardError
+  end
+end