check for errors messages instead of direct save

[itsho]

This task downloads files but sometimes, TFS could throw an error, and this is will - mistakenly, get saved as the requested file.

for example - the content could be this:

 {"$id":"1","innerException":null,"message":"TF15004: The download request signature has 
expired.","typeName":"Microsoft.TeamFoundation.Framework.Server.DownloadTicketValidationException, 
Microsoft.TeamFoundation.Framework.Server","typeKey":"DownloadTicketValidationException","errorCode":0,"eventId":4008}