Throw exception if user not found

If somehow the user that matches the identity has been deleted, throw
an exception instead of returning a null user.

Resolves #49.

src/UserStore.php

@@ -22,6 +22,6 @@ public function store($user)
 
     public function findByIdentity($identity)
     {
-        return $identity->belongsTo($this->model, 'user_id')->first();
+        return $identity->belongsTo($this->model, 'user_id')->firstOrFail();
     }
 }