UPGRADE: mpr, http paks.

    REFACTOR: Change HttpStage open to take a return status code.

    If HttpStage.open gets an error, its close() callback should not be invoked. To handle this, open must return a stat
    If the handler open() calls httpError, the passHandler needs to be used instead thereafter.
    Also need more robust pendingFinalize if httpError is called before the pipeline is fully constructed.