Add details to HttpException string

Without this change:

    400 Client Error: Bad Request

With this change, the much more useful:

    HttpException: 400 Client Error: Bad Request, Validation error: instance 'flavorRef' is a required property

Change-Id: Iaa63969e327532c02d72999efcbee42aa0d381d3
Merge "Add details to HttpException string"