The middleware pipeline should halt on error

Right now, when next(err) is called, the pipeline will switch into an "error mode" and start to process the error down the error middleware. If some middleware then calls next() (i.e. double-next) then the normal pipeline processing will resume.

I think that once an err has been provided to the pipeline, a flag should be set in the pipeline and simply ignore all next() calls (non-error-pipeline calls). This would probably be a sane way to manage errors, as it would be an analogue to someone raising a flag when they get on('error', fn) called and want to ignore the results from non-cancel-able async operations.