add error handler pages for 400, 404, 500

tests/functional_tests.py

@@ -10,15 +10,24 @@ def test_page_that_exists(client):
 
 
 def test_page_that_doesnt_exist(client):
-    """Test that the app returns 404 for nonsense requests."""
+    """Test that the app returns 404 for nonsense requests and they use my error page."""
     response = client.get('/ohuesthaoeusth')
     assert response.status_code == 404
+    assert b'<b><tt>/ohuesthaoeusth</tt></b> does not seem to exist' in response.data
 
 
 def test_files_outside_pages_do_not_get_served(client):
-    """Test that page pathing doesn't break out of the instance/pages/ dir."""
+    """Test that page pathing doesn't break out of the instance/pages/ dir, and the error uses my error page."""
     response = client.get('/../unreachable')
     assert response.status_code == 400
+    assert b'You\'re doing something you\'re not supposed to. Stop it?' in response.data
+
+
+def test_internal_server_error_serves_error_page(client):
+    """Test that various exceptions serve up the 500 page."""
+    response = client.get('/actually-a-png')
+    assert response.status_code == 500
+    assert b'INTERNAL SERVER ERROR' in response.data
 
 
 def test_weird_paths_do_not_get_served(client):