Respond to proxy requests properly (try fix #1212)

Try to fix #1212 by responding to a proxy request with an `Invalid Request` error on the browser end instead of no error and an error on the server logs.
5xx usually means something is wrong on the server; 4xx means the client did something wrong.  400 seems right to me.

Not sure why you are using text/html when later we have a text/html specifying charset.  Maybe just keep it short and sweet like the 404 handler lower in the file.