fix: Task failure errors include stack of running tasks.

[GrahamDennis]

Previously if a task was run as a dependency of another task, the error message simply reported something like:

exit status 1

It is desirable instead to name the root task and all child tasks in the tree to the failing task.

After this PR, the error message will read:

task: Failed to run task "root": task: Failed to run task "failing-task": exit status 1

Additionally, task errors did not use the task's label name when present, which they now will.