/login/: Remove previous alerts before displaying new alerts.

The issue is that the server wants to display an error when you make a
request and fail, but the jQuery form validator also wants to display
errors at the same time. This removes the server errors to display the
jQuery.

Fixes: #8239.

static/js/portico/signup.js

@@ -82,5 +82,10 @@ $(function () {
 
             form.submit();
         },
+        invalidHandler: function () {
+            // this removes all previous errors that were put on screen
+            // by the server.
+            $("#login_form .alert.alert-error").remove();
+        },
     });
 });