message: update_agent: log deploy attempt outcome
  This logs the result of a deploy attempt, which is currently visible
  as service status only.
diff: @@ -298,18 +298,22 @@ impl UpdateAgent {
  let state_change = deploy_outcome.map(move |res, actor, _ctx| {
  match res {
  Ok(_) => {
  - update_unit_status(&format!("update staged: {}", release.version));
  + let msg = format!("update staged: {}", release.version);
  + update_unit_status(&msg);
  + log::trace!("{}", msg);
  actor.state.update_staged(release);
  }
  Err(_) => {
  let release_ver = release.version.clone();
  let fail_count = actor.deploy_attempt_failed(release);
  - update_unit_status(&format!(
  + let msg = format!(
  "trying to stage {} ({} failed deployment attempt{})",
  release_ver,
  fail_count,
  if fail_count > 1 { "s" } else { "" }
  - ));
  + );
  + update_unit_status(&msg);
  + log::trace!("{}", msg);
  }
  };
  Ok(())
  