fix: Import missing function in server_error template.

cms/templates/error.html

@@ -4,6 +4,7 @@
 from django.utils.translation import ugettext as _
 from django.urls import reverse
 from django.conf import settings
+from openedx.core.djangolib.markup import Text
 %>
 <%block name="bodyclass">error</%block>
 <%block name="title">