No tracebacks on normal errors and prettier error pages (#1768)

* Default error handler now only logs traceback on 500 errors and all responses are HTML formatted.

* Tests passing.

* Ability to flag any exception object with self.quiet = True following @ashleysommer suggestion.

* Refactor HTML formatting into errorpages.py. String escapes for debug tracebacks.

* Remove extra includes

* Auto-set quiet flag also when decorator is used.

* Cleanup, make error pages (probably) HTML5-compliant and similar for debug and non-debug modes.

* Fix lookup of non-existant status codes

* No logging of 503 errors after all.

* lint

tests/test_app.py

@@ -105,10 +105,7 @@ def test_app_handle_request_handler_is_none(app, monkeypatch):
 
     request, response = app.test_client.get("/test")
 
-    assert (
-        response.text
-        == "Error: 'None' was returned while requesting a handler from the router"
-    )
+    assert "'None' was returned while requesting a handler from the router" in response.text
 
 
 @pytest.mark.parametrize("websocket_enabled", [True, False])
@@ -189,7 +186,7 @@ def test_handle_request_with_nested_sanic_exception(app, monkeypatch, caplog):
     with caplog.at_level(logging.ERROR):
         request, response = app.test_client.get("/")
     assert response.status == 500
-    assert response.text == "Error: Mock SanicException"
+    assert "Mock SanicException" in response.text
     assert (
         "sanic.root",
         logging.ERROR,