message: Don't print a download summary if an error happens
  Otherwise it sort of just clutters things up!
diff: @@ -264,6 +264,7 @@ pub struct Downloads<'a, 'cfg: 'a> {
  downloaded_bytes: u64,
  largest: (u64, String),
  start: Instant,
  + success: bool,
  }
  struct Download {
  @@ -323,6 +324,7 @@ impl<'cfg> PackageSet<'cfg> {
  downloads_finished: 0,
  downloaded_bytes: 0,
  largest: (0, String::new()),
  + success: false,
  })
  }
  @@ -341,6 +343,7 @@ impl<'cfg> PackageSet<'cfg> {
  while downloads.remaining() > 0 {
  pkgs.push(downloads.wait()?);
  }
  + downloads.success = true;
  Ok(pkgs)
  }
  @@ -651,9 +654,14 @@ impl<'a, 'cfg> Drop for Downloads<'a, 'cfg> {
  if !progress.is_enabled() {
  return
  }
  + // If we didn't download anything, no need for a summary
  if self.downloads_finished == 0 {
  return
  }
  + // If an error happened, let's not clutter up the output
  + if !self.success {
  + return
  + }
  let mut status = format!("{} crates ({}) in {}",
  self.downloads_finished,
  ByteSize(self.downloaded_bytes),
  