feat: generic blueprint HTTP 404 error handler

This allows blueprint to handle 404 errors. This is not supported
directly in Flask.
https://flask.palletsprojects.com/en/stable/errorhandling/#handling
    However, the blueprint cannot handle 404 routing errors because the
    404 occurs at the routing level before the blueprint can be determined.

canaille/app/flask.py

@@ -5,7 +5,9 @@ from urllib.parse import urlunsplit
 
 from flask import abort
 from flask import current_app
+from flask import make_response
 from flask import request
+from werkzeug.exceptions import HTTPException
 from werkzeug.routing import BaseConverter
 
 from canaille.app.i18n import gettext as _
@@ -107,3 +109,19 @@ def model_converter(model):
             return instance
 
     return ModelConverter
+
+
+def redirect_to_bp_handlers(app, error):
+    """Find and execute blueprint handlers matching an error.
+
+    There is currently no way to make 404 handling generic:
+    https://flask.palletsprojects.com/en/stable/errorhandling/#handling
+        However, the blueprint cannot handle 404 routing errors because the
+        404 occurs at the routing level before the blueprint can be determined.
+    """
+    for bp in app.blueprints.values():
+        if bp.url_prefix and request.path.startswith(bp.url_prefix):
+            for type_, handler in bp.error_handler_spec[None][None].items():
+                if type_ in (error.code, HTTPException):  # pragma: no branch
+                    return make_response(handler(error))
+    return None

canaille/app/themes.py

@@ -40,15 +40,11 @@ if flask_themer:
 
         @app.errorhandler(404)
         def page_not_found(error):
-            # There is currently no way to make 404 handling generic
-            # https://flask.palletsprojects.com/en/stable/errorhandling/#handling
-            #     However, the blueprint cannot handle 404 routing errors because the
-            #     404 occurs at the routing level before the blueprint can be determined.
-            if flask.request.path.startswith("/scim/"):
-                from canaille.scim.endpoints import http_error_handler
+            from canaille.app.flask import redirect_to_bp_handlers
 
-                return http_error_handler(error)
-            return render_template("error.html", description=error, error_code=404), 404
+            return redirect_to_bp_handlers(app, error) or render_template(
+                "error.html", description=error, error_code=404
+            ), 404
 
         @app.errorhandler(500)
         def server_error(error):  # pragma: no cover
@@ -58,4 +54,9 @@ else:  # pragma: no cover
     render_template = flask.render_template
 
     def setup_themer(app):
-        return
+        @app.errorhandler(404)
+        def page_not_found(error):
+            from canaille.app.flask import redirect_to_bp_handlers
+
+            if not redirect_to_bp_handlers(app, error):
+                raise error