Report reason for cancellation of parallel task

[aucampia]

When two tasks run in parallel, and one fails, the other seems to be cancelled, but this cancellation is not reported explicitly.

It would be significantly clearer to users what is happening if the reason for cancellation was explicitly reported.

So for example, instead of (GitHub Actions output):

⯈lint:mypy
⯈CI:test
task: Failed to run task "CI:lint": task: Failed to run task "lint:mypy": exit status 1
Error: Process completed with exit code 201.

have

⯈lint:mypy
⯈CI:test
task: Terminated `CI:test` because parallel running task `CI:lint` failed.
task: Failed to run task "CI:lint": task: Failed to run task "lint:mypy": exit status 1
Error: Process completed with exit code 201.