var HttpError = function(code, message){
  Error.call(this, message);
  err.code = code;
}
HttpError.prototype = Object.create(Error.prototype);
HttpError.prototype.code = 500;
HttpError.prototype.applyToRes = function(res){
   res.status(this.code).send(this.message);
   return res;
}

  metaEngine.getAsync( id ).then( function( meta ){
    console.log( 'success!' )
    res.set( 'Content-Type', meta.mimeType )
    res.set( 'Content-Length', file.length )
    return storageEngine.getAsync( id )
  }, function(){
    console.log( 'failure!' )
    throw new HttpError(404, "Not found");
  } ).then( function( file ){
    console.log( 'still running' )
    if( file.pipe ){
      file.pipe( res )
      return
    } else if( typeof file === 'string' || Buffer.isBuffer( file ) ){
      res.send( file )
    } else {
      throw new HttpError(500, 'Invalid file returned. Please return ReadableStream, string, or Buffer' )
    }
  })
  .catch(HttpError, function(err){
    err.applyToRes(res);
  })
  .catch(function(err){
    new HttpError(500, err.message).applyToRes(res);
  });