message: List URL in HTTP download failures
  Hopefully will help diagnose issues like
diff: @@ -263,7 +263,9 @@ impl<'cfg> RegistryData for RemoteRegistry<'cfg> {
  body.extend_from_slice(buf);
  Ok(buf.len())
  })?;
  - handle.perform()?;
  + handle.perform().chain_err(|| {
  + format!("failed to download from `{}`", url)
  + })?;
  }
  let code = handle.response_code()?;
  if code != 200 && code != 0 {
  