mostrar el mensaje de error apropiado al usuario de la llamada Meteor

I am working with Meteorjs First time. In my application I am trying to Register a use. For this I am making a meteor call. Which returns me error if it is unsuccessful or success if user is registered successfully. My problem is when Someone try to register a user with a previously registered id meteor call returns error message that user name is already exists.this is my meteor call

var acct = {"email": email, "full_name": full_name, roles: roles}
var id = Meteor.call("register_staff", acct, function (error, result) {
if (error) {
     console.log(error)
     FlashMessages.sendError(" This email address is already registered")
} 
else {
       $("#user_creator_dialog").modal('hide');
       FlashMessages.sendSuccess("User Successesfully Created and Password Mailed")
     }
  }); 

si lo hago

 console.log(error)  

it shows me a Key value pair

Error: Username is already exists.

But when I try to get the key by using dot(.) notation like

console.log(error.Error)

it gives undefined

. my question is how can i access this error object key. before this i worked with javascript and jquery where dot notation works perfectly. I think in meteor there may be some other way to access this key.