feat: Add paths to easily view various error pages.

Currently it's hard to see the content of an error without knowing how
to cause an existing view to make that error in production.  Adding
these default paths should make that a lot easier.

lms/urls.py

@@ -7,6 +7,7 @@ from django.conf import settings
 from django.conf.urls import include, url
 from django.conf.urls.static import static
 from django.contrib.admin import autodiscover as django_autodiscover
+from django.urls import path
 from django.utils.translation import ugettext_lazy as _
 from django.views.generic.base import RedirectView
 from edx_api_doc_tools import make_docs_urls
@@ -71,6 +72,7 @@ if settings.DEBUG or settings.FEATURES.get('ENABLE_DJANGO_ADMIN_SITE'):
 # pylint: disable=invalid-name
 handler403 = static_template_view_views.render_403
 handler404 = static_template_view_views.render_404
+handler429 = static_template_view_views.render_429
 handler500 = static_template_view_views.render_500
 
 notification_prefs_urls = [
@@ -199,6 +201,10 @@ urlpatterns = [
     ),
     url(r'^api/discounts/', include(('openedx.features.discounts.urls', 'openedx.features.discounts'),
                                     namespace='api_discounts')),
+    path('403', handler403),
+    path('404', handler404),
+    path('429', handler429),
+    path('500', handler500),
 ]
 
 if settings.FEATURES.get('ENABLE_MOBILE_REST_API'):