Catch non json results when verifying election

If you try to verify a election and you aren't allowed, you get the following
error. This is because it is expected a JSON result and you get, for instance,
a html page to login.

VM65:1 Uncaught SyntaxError: Unexpected token < in JSON at position 0
    at JSON.parse (<anonymous>)
    at Function.HELIOS.Election.fromJSONString (helios.js:127)
    at Object.success (verify.html?election_url=/helios/elections/258ca6b1-afa4-49a3-b484-c31679d2ebbe:100)
    at success (jquery-1.2.2.min.js:28)
    at onreadystatechange (jquery-1.2.2.min.js:27)