title: Vee validate message in an extend async rule fails to show error message
body: <p>Am using vee validate with vuetify and i need to check if email is unique. 
  So in my vuejs i have added</p>
  
  <pre><code>mounted(){
  
         extend('unique-email', (value) =&gt; {
           return this.$axios.post('/api/auth/unique-email', { email: value })
           .then((res) =&gt; {
            return {
              valid: true,
            };
          }, (err) =&gt; {
            return {
              valid: false,
              data: { message: 'Email already registered' }
            };
          })
          }, {
           immediate: false
         })
     }
  </code></pre>
  
  <p>In my vuetify i have added</p>
  
  <pre><code>&lt;v-textfield v-model="form.email" rules="required|email|unique-emai"&gt;
  </code></pre>
  
  <p>The above works for all the rules but doesnt resolve the message <code>email already registred</code> from the unique-email rule. What do i need to add so that if the async validation fails the message from the err part is displayed. </p>
  
  <p>Currently it only shows <code>email is not valid</code> message when the unique-email validator fails.
  What am i missing?</p>