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ARROW-8371: [Crossbow] Implement and exercise sanity checks for tasks.yml

- implement `python crossbow.py check-config`
- also run the check before job submission
- a bit more verbose error messages for template rendering issues
- run these sanity checks if anything changes under dev/tasks

Closes #6875 from kszucs/crossbow-check-config

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