ARROW-16252: [CI][Archery] Highlight number of failed builds on nightly reports
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 <59d4da135a4a8d271cfab38e4b71ac8d4bb78fb2@gmail.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

apache/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,icexelloss/arrow,icexelloss/arrow,kou/arrow,kou/arrow,wesm/arrow,wesm/arrow,apache/arrow,icexelloss/arrow,apache/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,icexelloss/arrow,apache/arrow,icexelloss/arrow,kou/arrow,wesm/arrow,kou/arrow,icexelloss/arrow,kou/arrow,kou/arrow,kou/arrow,icexelloss/arrow,apache/arrow,apache/arrow,apache/arrow,wesm/arrow,apache/arrow,kou/arrow,icexelloss/arrow,wesm/arrow,wesm/arrow,kou/arrow,apache/arrow,apache/arrow,wesm/arrow,kou/arrow,wesm/arrow,kou/arrow,wesm/arrow,icexelloss/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,kou/arrow,icexelloss/arrow,kou/arrow,apache/arrow,wesm/arrow,icexelloss/arrow,wesm/arrow,kou/arrow