Fix FlaskApp exception handlers (#1788)

Fixes #1787 

In the `FlaskApp`, the error handlers were still registered on the
underlying Flask app instead of on the `ExceptionMiddleware`, which led
to them not following the documented behavior.

The documentation was also incorrect about ignoring error handlers
registered on the flask application. We are only ignoring the default
error handlers registered by Flask itself.

This is a breaking change, however, since this functionality was not
following the documented behavior, and 3.0.0 was only recently released,
I propose to release this as a patch version.

tests/api/test_bootstrap.py

@@ -1,3 +1,4 @@
+import json
 from unittest import mock
 
 import jinja2
@@ -7,6 +8,7 @@ from connexion import App
 from connexion.exceptions import InvalidSpecification
 from connexion.http_facts import METHODS
 from connexion.json_schema import ExtendedSafeLoader
+from connexion.lifecycle import ConnexionRequest, ConnexionResponse
 from connexion.middleware.abstract import AbstractRoutingAPI
 from connexion.options import SwaggerUIOptions
 
@@ -302,10 +304,15 @@ def test_add_error_handler(app_class, simple_api_spec_dir):
     app = app_class(__name__, specification_dir=simple_api_spec_dir)
     app.add_api("openapi.yaml")
 
-    def custom_error_handler(_request, _exception):
-        pass
+    def not_found(request: ConnexionRequest, exc: Exception) -> ConnexionResponse:
+        return ConnexionResponse(
+            status_code=404, body=json.dumps({"error": "NotFound"})
+        )
 
-    app.add_error_handler(Exception, custom_error_handler)
-    app.add_error_handler(500, custom_error_handler)
+    app.add_error_handler(404, not_found)
 
-    app.middleware._build_middleware_stack()
+    app_client = app.test_client()
+
+    response = app_client.get("/does_not_exist")
+    assert response.status_code == 404
+    assert response.json()["error"] == "NotFound"