Wrap error codes into 400-600 range

[back2dos]

When generating status codes from error codes, it should be % 200 + 400, to make sure it always comes out as an error code. It will thus be possible to introduce new error codes outside the HTTP range and let them be mapped to HTTP error codes nicely.

[kevinresol]

Related: 13413e2

I added that because nodejs will crash when the status code is outside such range.