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_response_timeout.py

@@ -40,7 +40,7 @@ async def handler_2(request):
 def test_default_server_error_response_timeout():
     request, response = response_timeout_default_app.test_client.get("/1")
     assert response.status == 503
-    assert response.text == "Error: Response Timeout"
+    assert "Response Timeout" in response.text
 
 
 response_handler_cancelled_app.flag = False
@@ -65,5 +65,5 @@ async def handler_3(request):
 def test_response_handler_cancelled():
     request, response = response_handler_cancelled_app.test_client.get("/1")
     assert response.status == 503
-    assert response.text == "Error: Response Timeout"
+    assert "Response Timeout" in response.text
     assert response_handler_cancelled_app.flag is False