ENYO-2788: [Do not merge] Check the existance of a package before downloading it from Build status UI

#### Goal
The purpose Of this pull request is to confirm the code design on the widget **Build Status**, especially on the part of checking the existence or not of the built package to be downloaded from this widget.

The rest of the description is in https://enyojs.atlassian.net/browse/ENYO-2788

Tested on Chrome Canary (Windows 7)
Enyo-DCO-1.1-Signed-off-by: Rafik Adiche mahmoud-rafik.adiche@hp.com