Dummy client: Better handling of tasks

In the dummy client the current task channel is implemented to handle a happy path but could be improved on error handling and task recycling when one of them fails.

We should make those improvement to make it production ready.

This applies for the `init` method and the handling of tasks in the `verifier_task`

Start by locating the dummy client's init method and verifier_task handling. Trace what happens when a task fails, then determine the expected error and recycling behavior; done means failed tasks are handled and reusable without breaking the task channel.

Written by the indexing model from the issue text.