Missing packages should return HTTP 404

[Rufflewind]

If https://crates.io/crates/nonexistent_package does not refer to a valid package, it should respond with HTTP 404 to avoid misleading crawlers and/or browsers into thinking that it's a valid page.

[onur]

It is responding 404:

$ curl -D- https://crates.io/crates/nonexistent_package
HTTP/1.1 404 Not Found
Connection: keep-alive
Server: nginx
Date: Mon, 13 Feb 2017 12:34:09 GMT
Content-Type: application/json; charset=utf-8
Content-Length: 35
Set-Cookie: cargo_session=--3Mz45cH3itfVkmY41+nVjWRm9+0=; HttpOnly; Secure; Path=/
Via: 1.1 vegur

{"errors":[{"detail":"Not Found"}]}

[Rufflewind]

@onur It seems to be dependent on the Accept header:

$ curl -v -H 'Accept: text/html' https://crates.io/crates/nonexistent_package
< …
< HTTP/1.1 200 OK
< …

[carols10cents]

Closing as a duplicate of #788.