Add stack trace to promises output.

This makes it much easier to debug when there are problems.

Before:

```
index.js:21 Failed to load the deck. TypeError: Cannot read property 'keys' of undefined(…)
```

After:

```
index.js:21 Failed to load the deck.
TypeError: Cannot read property 'keys' of undefined(…)
TypeError: Cannot read property 'keys' of undefined
     at wgf.getDeck
     (http://localhost:8080/static/js/wgf.js:73:28)(anonymous function)
     @ index.js:21
```
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

// TODO: browserfy var wgf = require('wgf')

if (location.hash) {
  var hashParams = location.hash.split('?')[1]
  CURRENT_LANGUAGE = hashParams.split('=')[1]
}

document.addEventListener(
  'DOMContentLoaded',
  function () {
    // TODO: make path to root relative
    wgf.listCards({rootPath: '', preferredLanguage: CURRENT_LANGUAGE})
      .then(wgf.getDeck)
      .then(wgf.attachNextCard)
      .catch(
        function (error) {
          console.error('Failed to load the deck.\n', error, '\n', error.stack)
        })
  })

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

// TODO: browserfy var wgf = require('wgf')

if (location.hash) {
  var hashParams = location.hash.split('?')[1]
  CURRENT_LANGUAGE = hashParams.split('=')[1]
}

document.addEventListener(
  'DOMContentLoaded',
  function () {
    // TODO: make path to root relative
    wgf.listCards({rootPath: '', preferredLanguage: CURRENT_LANGUAGE})
      .then(wgf.getDeck)
      .then(
        wgf.attachNextCard,
        function (error) {
          console.error('Failed to load the deck.', error)
        })
  })
