getCurrent url bug

I just fixed the issue #63, that created a bad HTTP request, with two slashes in the URL instead of one.
```js
   .then(function(){}, function() {
     throw new Error('It should not fail');
   });
```

This will print "it should not fail" messages without any details, which makes troubleshooting very difficult. Use `throwHttpError` helper instead:

```js
return givenLoggedInUser()
  .then(function(){
    return Customer.getCurrent().$promise;
  })
 .catch(util.throwHttpError);
```