Show an error for incomplete downloads

[ducaale]

This is how HTTPie 3.1.1.dev0 handles incomplete downloads:

http -d httpbin.org/gzip
HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Content-Encoding: gzip
Content-Length: 224
Content-Type: application/json
Date: Sat, 14 May 2022 12:04:33 GMT
Server: gunicorn/19.9.0

Downloading to gzip.json
Done. 284 bytes in 00:0.18373 (1.5 kB/s)  # "Done" becomes "Interrupted" when there is an actual error

http: error: Incomplete download: size=224; downloaded=284

This can be a little bit tricky when resuming an interrupted download via --continue or when dealing with compressed responses.