fix: do not parse return value of errored tasks - #692

When a task fails, the receiver stores a result with `is_err=True` and
`return_value=None`. `AsyncTaskiqTask.get_result` still tried to validate
that `None` against the task's return annotation, so every failed task
with a non-optional return type logged a misleading
"Cannot parse return type into ..." warning on the client side.

Skip the validation when the result is an error: there is no return value
to parse, and the actual failure is already available in `result.error`.

Closes: taskiq-python#479