LibWeb: Implement the "error to rethrow" mechanism in HTML::Script

This allows JS module loads to fail and throw without crashing the
WebContent process due to a TODO() assertion.

Userland/Libraries/LibWeb/HTML/Scripting/ClassicScript.h

@@ -43,7 +43,6 @@ private:
 
     JS::GCPtr<JS::Script> m_script_record;
     MutedErrors m_muted_errors { MutedErrors::No };
-    Optional<JS::ParserError> m_error_to_rethrow;
 };
 
 }