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

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.title.string.strip() == 'Bad request – GOV.UK Notify'
+    assert page.h1.string.strip() == 'Sorry, there’s a problem with 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.title.string.strip() == 'Bad request – GOV.UK Notify'
+    assert page.h1.string.strip() == 'Sorry, there’s a problem with 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,
     )