Add a custom error for task not found errors

[Johannes Maron]

Regression from #36919

When a user renames or moves a task, deconstruction fails with an ambiguous ValueError shadowing the ImportError.

​https://github.com/django/django/blob/cf564bf3a7e0d92bc32292ccff1eca5aeb24b1ef/django/tasks/base.py#L64-L66
shadows ImportError.

Currently you'd need to access causes to figure out that the ValueError was actually because the tasks can't be found.

It would be more suitable to fail with a custom error, which backends catch and handle more gracefully.

[Johannes Maron]

I would propose something like:

class TaskNotFoundException(ValueError):
    def __init__(self, func_path, *args, **kwargs):
        self.func_path = func_path
        super().__init__(f"Expected {func_path!r} to point to a Task instance.")

[Johannes Maron]

BTW, the message suggests that we did a type-check, which don't :shrug:

[Sarah Boyce]

I think a custom error is reasonable. Needs docs similar to TaskError (and a versionadded note etc)

[Jake Howard]

I'd suggest naming it TaskDoesNotExist to mirror the existing TaskResultDoesNotExist, and match the similar ORM exceptions. There's already InvalidTask, but this feels sufficiently different to warrant its own exception.

[Johannes Maron]

TaskDoesNotExist sounds great, I'll whip up a small patch.