Handle login error

```js
Hull.login('facebook').then(function(user) {
  console.log('Hello ' + user.name);
}, function(error) {
  if (error.reason === 'email_taken') {
    console.log(error.email + ' is already taken by user with id: ' + error.user_id)
} else if (error.reason === 'auth_failed') {
    console.log('User did not fully authorize or provider app is not well configured');  
} else if (error.reason === 'window_closed') {
    console.log('User closed window');
  } else {
    console.log('Something when wrong on Hull side...');
  }
});
```

```js
Hull.linkIdentity('twitter').then(function() {
  console.log('Twitter identity has been linked');
}, function(error) {  
  if (error.reason == 'identity_taken') {
    console.log('Twitter identity is already linked to another user');
  } else {
    console.log('Something when wrong on Hull side...');
  }
});
```

```js
Hull.unlinkIdentity('twitter').then(function() {
  alert('Twitter identity has been unlinked');
}, function(error) {
  console.log(error.message);
});
```

```js
Hull.logout().then(function() {
  console.log('Goodbye');
});
```