Issue Description:

On Linux (tested with swift 5.1.4) the following (just at the lldb prompt will do) returns a valid chunk of data with no exception thrown. The data, if converted to a string, is Apple's 404 page.

(lldb) p Data(contentsOf:URL(string:"https://www.apple.com/a404/")!)

(Data) $R14 = 35006 bytes

On MacOS the same thing throws an exception.

I think (and I'm looking at _NSNonfileURLContentLoader) that it's just not inspecting the http response status codes. Given that there's no way for my app to know it's a 404 by inspecting the returned Data object this is incorrect behaviour.

For completeness curl confirms it's a 404 🙂

$ curl https://www.apple.com/a404/ -vvvv

...snip..

• Connection state changed (MAX_CONCURRENT_STREAMS == 100)!
  < HTTP/2 404
  < server: Apache
  < x-frame-options: SAMEORIGIN
  < x-xss-protection: 1; mode=block
  < x-content-type-options: nosniff
  <