remove the generic 400 error page handler

it just shows a h1, so isn't helpful for people. We can re-use the 500
error page, which includes instructings "Try again later" and
instructions on what to do next (check the status page, email notify
support).

this required refactoring to ensure we can show the 500 error page while
still returning the required status code

app/__init__.py

@@ -1,4 +1,3 @@
-import itertools
 import os
 import urllib
 from datetime import datetime, timedelta, timezone
@@ -602,8 +601,10 @@ def useful_headers_after_request(response):
 
 
 def register_errorhandlers(application):  # noqa (C901 too complex)
-    def _error_response(error_code):
+    def _error_response(error_code, error_page_template=None):
-        resp = make_response(render_template("error/{0}.html".format(error_code)), error_code)
+        if error_page_template is None:
+            error_page_template = error_code
+        resp = make_response(render_template("error/{0}.html".format(error_page_template)), error_code)
         return useful_headers_after_request(resp)
 
     @application.errorhandler(HTTPError)
@@ -627,8 +628,10 @@ def register_errorhandlers(application):  # noqa (C901 too complex)
         return _error_response(error_code)
 
     @application.errorhandler(400)
-    def handle_400(error):
+    def handle_client_error(error):
-        return _error_response(400)
+        # This is tripped if we call `abort(400)`.
+        application.logger.exception('Unhandled 400 client error')
+        return _error_response(400, error_page_template=500)
 
     @application.errorhandler(410)
     def handle_gone(error):
@@ -671,19 +674,11 @@ def register_errorhandlers(application):  # noqa (C901 too complex)
             u'csrf.invalid_token: Aborting request, user_id: {user_id}',
             extra={'user_id': session['user_id']})
 
-        resp = make_response(render_template(
+        return _error_response(400, error_page_template=500)
-            "error/400.html",
-            message=['Something went wrong, please go back and try again.']
-        ), 400)
-        return useful_headers_after_request(resp)
 
     @application.errorhandler(405)
-    def handle_405(error):
+    def handle_method_not_allowed(error):
-        resp = make_response(render_template(
+        return _error_response(405, error_page_template=500)
-            "error/400.html",
-            message=['Something went wrong, please go back and try again.']
-        ), 405)
-        return useful_headers_after_request(resp)
 
     @application.errorhandler(WerkzeugHTTPException)
     def handle_http_error(error):

app/main/views/uploads.py

@@ -234,7 +234,6 @@ def uploaded_letter_preview(service_id, file_id):
 @main.route("/services/<uuid:service_id>/preview-letter-image/<uuid:file_id>")
 @user_has_permissions('send_messages')
 def view_letter_upload_as_preview(service_id, file_id):
-
     try:
         page = int(request.args.get('page'))
     except ValueError:

app/templates/error/400.html

@@ -1,11 +0,0 @@
-{% extends "withoutnav_template.html" %}
-{% block per_page_title %}Bad request{% endblock %}
-{% block maincolumn_content %}
-<div class="grid-row">
-    <div class="column-two-thirds">
-      <h1 class="heading-medium">
-            {{ message|join(",")}}
-        </h1>
-    </div>
-</div>
-{% endblock %}

tests/app/main/test_errorhandlers.py

@@ -51,8 +51,8 @@ def test_csrf_returns_400(logged_in_client, mocker):
 
     assert response.status_code == 400
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
-    assert page.h1.string.strip() == 'Something went wrong, please go back and try again.'
+    assert page.h1.string.strip() == 'Sorry, there’s a problem with GOV.UK Notify'
-    assert page.title.string.strip() == 'Bad request – GOV.UK Notify'
+    assert page.title.string.strip() == 'Sorry, there’s a problem with the service – GOV.UK Notify'
 
 
 def test_csrf_redirects_to_sign_in_page_if_not_signed_in(client, mocker):
@@ -70,5 +70,5 @@ def test_405_returns_something_went_wrong_page(client, mocker):
 
     assert response.status_code == 405
     page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
-    assert page.h1.string.strip() == 'Something went wrong, please go back and try again.'
+    assert page.h1.string.strip() == 'Sorry, there’s a problem with GOV.UK Notify'
-    assert page.title.string.strip() == 'Bad request – GOV.UK Notify'
+    assert page.title.string.strip() == 'Sorry, there’s a problem with the service – GOV.UK Notify'

tests/app/main/views/test_uploads.py

@@ -504,6 +504,7 @@ def test_uploaded_letter_preview_image_400s_for_bad_page_type(
         file_id=fake_uuid,
         service_id=SERVICE_ONE_ID,
         page='foo',
+        _test_page_title=False,
         _expected_status=400,
     )