Adds additional tests on json raw data for parsing

When testing a deployment of Thinktecture.IdentityServerv2 I forgot to add
my user before running a call to OAuth2Client.RequestResourceOwnerPassword
method. If you forget to add the user like I did this is the response back
from the server's endpoint.
"{ Message: { \"error\": \"invalid_client\" }}"

This pull request will add additional checks for messages like that so
that the calling application can use the pre-existing token.IsError
property.