If instantiation has failed, then also fail the load and don't fetch imports. Ref BZ: 1358882
This splits ScriptLoader up the same way Mozilla did with the exception of ScriptRequest due to the fact that ScriptLoader and ScriptRequest are interdependent and would create a circular dependency if split apart when not using unified building.
- Moves scripting parts of DOM into 'dom/script' - Renames nsScript{Loader/Element} to Script{Loader/Element} - Adjusts all callers