flows: planner error handling (#4812)

* handle FlowNonApplicableException everywhere

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* make flow planner check authentication when no pending user is in planning context

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

* add mailhog to e2e test services, remove local docker requirement

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

---------

Signed-off-by: Jens Langhammer <jens@goauthentik.io>

authentik/sources/saml/views.py

@@ -22,6 +22,7 @@ from authentik.flows.challenge import (
     ChallengeResponse,
     ChallengeTypes,
 )
+from authentik.flows.exceptions import FlowNonApplicableException
 from authentik.flows.models import in_memory_stage
 from authentik.flows.planner import (
     PLAN_CONTEXT_REDIRECT,
@@ -87,7 +88,10 @@ class InitiateView(View):
         # We run the Flow planner here so we can pass the Pending user in the context
         planner = FlowPlanner(source.pre_authentication_flow)
         planner.allow_empty_flows = True
-        plan = planner.plan(self.request, kwargs)
+        try:
+            plan = planner.plan(self.request, kwargs)
+        except FlowNonApplicableException:
+            raise Http404
         for stage in stages_to_append:
             plan.append_stage(stage)
         self.request.session[SESSION_KEY_PLAN] = plan