fix: ensure errors are not missed when creating tasks

[bhearsum]

I noticed this as an intermittent test failure in CI, which looks like a minor regression from #873:

[task 2026-06-29T11:51:43.891+00:00] ______________ TestCreate.test_create_tasks_fails_if_create_fails ______________
[task 2026-06-29T11:51:43.891+00:00] 
[task 2026-06-29T11:51:43.891+00:00] self = <test.test_create.TestCreate testMethod=test_create_tasks_fails_if_create_fails>
[task 2026-06-29T11:51:43.891+00:00] 
[task 2026-06-29T11:51:43.891+00:00]     @responses.activate
[task 2026-06-29T11:51:43.891+00:00]     @mock.patch.dict(
[task 2026-06-29T11:51:43.891+00:00]         "os.environ",
[task 2026-06-29T11:51:43.891+00:00]         {"TASKCLUSTER_ROOT_URL": "https://tc.example.com"},
[task 2026-06-29T11:51:43.891+00:00]         clear=True,
[task 2026-06-29T11:51:43.891+00:00]     )
[task 2026-06-29T11:51:43.891+00:00]     def test_create_tasks_fails_if_create_fails(self):
[task 2026-06-29T11:51:43.891+00:00]         "create_tasks fails if a single create_task call fails"
[task 2026-06-29T11:51:43.891+00:00]         mock_taskcluster_api(error_status=403, error_message="oh no!")
[task 2026-06-29T11:51:43.891+00:00]     
[task 2026-06-29T11:51:43.891+00:00]         tasks = {
[task 2026-06-29T11:51:43.891+00:00]             "tid-a": Task(
[task 2026-06-29T11:51:43.891+00:00]                 kind="test", label="a", attributes={}, task={"payload": "hello world"}
[task 2026-06-29T11:51:43.891+00:00]             ),
[task 2026-06-29T11:51:43.891+00:00]         }
[task 2026-06-29T11:51:43.891+00:00]         label_to_taskid = {"a": "tid-a"}
[task 2026-06-29T11:51:43.891+00:00]         graph = Graph(nodes={"tid-a"}, edges=set())
[task 2026-06-29T11:51:43.891+00:00]         taskgraph = TaskGraph(tasks, graph)
[task 2026-06-29T11:51:43.891+00:00]     
[task 2026-06-29T11:51:43.891+00:00] >       with self.assertRaises(CreateTasksException):
[task 2026-06-29T11:51:43.891+00:00]              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[task 2026-06-29T11:51:43.891+00:00] E       AssertionError: CreateTasksException not raised

It's probably impossible to hit in any non-trivial taskgraph in production, but it's certainly something that could happen, particularly in smaller/shallower graphs.

The fix is moving the error checking out to the main flow of control, and doing it synchronously. This leaves handle_exception doing nothing except populated skipped, so I moved it into schedule_tasks and renamed it to make that a bit more obvious.

[ahal]

Oops, thanks for catching this!