Validate auth field name if using arborist for auth

[qingyashu]

Right now the auth helper is casting an additional auth layer to the filter for accessing accessible datasets. But it doesn't have actually validate the field exists. So if guppy is mis-configured with wrong auth field name, the server won't go down, and graphql endpoint will always return BAD_USER_INPUT err, which is not helpful. Please add validation for auth field during initialization.