Display specific error messages on the login screen.

This allows us to inform users that the reason they are unable to log in
is because their account has been deactivated. This message (generated by
Django) is only shown when the correct credentials are used and is_active
is False.

(imported from commit 754a4c9af6ec05e8f18cf183bc08b67bacdeb3e0)

templates/zephyr/login.html

@@ -16,7 +16,9 @@ autofocus('#id_username');
 
 {% if form.errors %}
 <div class="alert alert-error">
-    Your email and password didn't match. Please try again.
+    {% for error in form.errors.values %}
+    <div>{{ error | striptags }}</div>
+    {% endfor %}
 </div>
 {% endif %}