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ARROW-16252: [CI][Archery] Highlight number of failed builds on nightly reports

This PR adds to the reports email subject the number of tasks that are pending and have failied.
Tested locally with a reduced number of tasks:
```bash
$ archery crossbow report --no-fetch nightly-packaging-2022-04-14-0
From: None <None>
To: None
Subject: [NIGHTLY] Arrow Build Report for Job nightly-packaging-2022-04-14-0: 2 failed, 0 pending

Arrow Build Report for Job nightly-packaging-2022-04-14-0

All tasks: https://github.com/raulcd/crossbow/branches/all?query=nightly-packaging-2022-04-14-0

Errored Tasks:
- debian-bookworm-arm64:
  URL: https://github.com/raulcd/crossbow/tree/nightly-packaging-2022-04-14-0-travis-debian-bookworm-arm64
- wheel-windows-cp310-amd64:
  URL: https://github.com/raulcd/crossbow/tree/nightly-packaging-2022-04-14-0-github-wheel-windows-cp310-amd64
```

Closes #12934 from raulcd/ARROW-16252

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>