♻️ Refactor out need for oidc error mapping

Discussed with Steven and Alex in the office, ultimately the hardcoded
approach is favourable so we can drop the error_mapping config field
entirely.

Some testing/tweaking may be needed to see if different brokers provide
the same error_description values, but adding the possible combinations
for all brokers is not even outrageous - there's only a handful of
such brokers in NL.

src/digid_eherkenning_oidc_generics/views.py

@@ -15,7 +15,6 @@
     _OIDC_ERROR_SESSION_KEY,
     OIDCAuthenticationCallbackView as BaseCallbackView,
     OIDCInit,
-    get_exception_message,
 )
 from solo.models import SingletonModel
 
@@ -60,6 +59,7 @@ def get(self, request):
 class OIDCCallbackView(BaseCallbackView):
     failure_url = reverse_lazy("oidc-error")
     generic_error_msg = ""
+    error_message_mapping: dict[tuple[str, str], str]
 
     def get(self, request):
         try:
@@ -70,26 +70,28 @@ def get(self, request):
                 "Something went wrong while attempting to authenticate via OIDC",
                 exc_info=exc,
             )
-            exc_message = get_exception_message(exc)
-            request.session[_OIDC_ERROR_SESSION_KEY] = exc_message
+            request.session[_OIDC_ERROR_SESSION_KEY] = str(self.generic_error_msg)
             response = self.login_failure()
         else:
             # Upstream library doesn't do any error handling by default.
             if _OIDC_ERROR_SESSION_KEY in request.session:
                 del request.session[_OIDC_ERROR_SESSION_KEY]
 
-        # XXX: move this to a separate model
-        error_message_mapping = self.get_settings("ERROR_MESSAGE_MAPPING")
-
-        error = request.GET.get("error_description")
-        error_label = error_message_mapping.get(error, str(self.generic_error_msg))
-        if error and error_label:
-            request.session[_OIDC_ERROR_SESSION_KEY] = error_label
-        elif _OIDC_ERROR_SESSION_KEY in request.session and error_label:
+        if error_label := self._map_error(request):
             request.session[_OIDC_ERROR_SESSION_KEY] = error_label
 
         return response
 
+    def _map_error(self, request) -> str:
+        if not (error := request.GET.get("error")):
+            return ""
+
+        error_description = request.GET.get("error_description", "")
+
+        # Look up the combination of error code and description in the mapping.
+        mapped_error = self.error_message_mapping.get((error, error_description))
+        return mapped_error or str(self.generic_error_msg)
+
 
 class BaseOIDCLogoutView(View):
     config_class: type[SingletonModel]
@@ -117,6 +119,11 @@ def get(self, request):
 # FIXME: mozilla-django-oidc-db has a proper construct for this now.
 class DigiDOIDCAuthenticationCallbackView(OIDCCallbackView):
     generic_error_msg = GENERIC_DIGID_ERROR_MSG
+    error_message_mapping = {
+        ("access_denied", "The user cancelled"): (
+            "Je hebt het inloggen met DigiD geannuleerd."
+        )
+    }
 
 
 class DigiDOIDCLogoutView(BaseOIDCLogoutView):
@@ -126,6 +133,11 @@ class DigiDOIDCLogoutView(BaseOIDCLogoutView):
 # FIXME: mozilla-django-oidc-db has a proper construct for this now.
 class eHerkenningOIDCAuthenticationCallbackView(OIDCCallbackView):
     generic_error_msg = GENERIC_EHERKENNING_ERROR_MSG
+    error_message_mapping = {
+        ("access_denied", "The user cancelled"): (
+            "Je hebt het inloggen met eHerkenning geannuleerd."
+        )
+    }
 
 
 class eHerkenningOIDCLogoutView(BaseOIDCLogoutView):