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)

heliosverifier/verify.html

@@ -97,6 +97,7 @@ function load_election_and_ballots(election_url) {
     
     // the hash will be computed within the setup function call now
     $.get(election_url, function(raw_json) {
+        try {
             election = HELIOS.Election.fromJSONString(raw_json);
             result_append("loaded election: " + election.name);
             result_append("election fingerprint: " + election.get_hash());
@@ -242,6 +243,12 @@ function load_election_and_ballots(election_url) {
                 });
 
             });
+    } catch (error) {
+        result_append("<p>It appears that you are trying to verify a private election.</p>");
+        result_append('<p>You can log in as a valid voter or log in as the election admin.</p>');
+        result_append('<a class="btn" href="' + election_url + '">Log in as a valid voter </a>');
+        result_append('<a class="btn" href="/auth/?return_url=/verifier/verify.html?election_url=' + election_url + '">Log in as the election admin</a>');
+    }
     });
         
 }