Fix error status codes in HTTP server

[confluence]

You can only set a status on a uWebSockets response once, and you have to set status first, then headers, then a body. To ensure that a status is set before headers are set, the uWebSockets function for setting headers automatically sets the status to 200 first. This means that any subsequent attempt to set a different status will have no effect.

In our server code, there are several places where we first set headers (through AddNoCacheHeaders or otherwise) and then attempt to set a status, which is silently failing. If the status is supposed to be 200 anyway, this makes no difference, but it's impossible for these functions ever to return an error status.

I found this while moving the scripting interface into the HTTP server, but I think that this needs to be fixed and tested in a separate PR -- it's possible that other parts of CARTA are depending on these functions always returning a 200 status, and that something will break once the error codes are restored.