Expose actual underlying error to calling methods

When calling a method such as `PlayFabClientAPI.LoginWithFacebook()` You can cause an HTTP error by doing the following, however the HTTP error won't propagate back to the calling method.

    LoginWithFacebookRequest request = new LoginWithFacebookRequest();
    ...
    request.TitleId = "";
    PlayFabClientAPI.LoginWithFacebook(request, OnLoginResult, OnPlayFabError);

Adding in this change will cause the error message to propagate forward appropriately, in this case a DNS error.