fix: make task failure errors include stack of running tasks (#2286)

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

errors/errors_task.go

@@ -54,6 +54,10 @@ func (err *TaskRunError) TaskExitCode() int {
 	return err.Code()
 }
 
+func (err *TaskRunError) Unwrap() error {
+	return err.Err
+}
+
 // TaskInternalError when the user attempts to invoke a task that is internal.
 type TaskInternalError struct {
 	TaskName string