ENYO-3735: Display an error message if the opened project doesn't contain the project.json file.

Link to the JIRA ticket: https://enyojs.atlassian.net/browse/ENYO-3735

Ready for review.

Tested on Chrome(Windows 7)
Enyo-DCO-1.1-Signed-off-by: Rafik Adiche mahmoud-rafik.adiche@hp.com