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

testdata/label_error/testdata/TestLabel-label_in_error.golden

@@ -0,0 +1 @@
+task: [foobar] false