Proper errorhandling for registration with invite

bookwyrm/templates/invite.html

@@ -3,7 +3,8 @@
 
 <div class="columns">
     <div class="column">
-        <div class="block login">
+        <div class="block">
+            {% if valid %}
             <h1 class="title">Create an Account</h1>
             <div>
                 <form name="register" method="post" action="/user-register">
@@ -11,6 +12,12 @@
                     {% include 'snippets/register_form.html' %}
                 </form>
             </div>
+            {% else %}
+            <div class="content">
+                <h1 class="title">Permission Denied</h1>
+                <p>Sorry! This invite code is no longer valid.</p>
+            </div>
+            {% endif %}
         </div>
     </div>
     <div class="column">