[sub]feat: modify computetask failure report (#727)

## Companion PR

- Substra/orchestrator#277
- Substra/substra-frontend#240

## Description

The aim is to allow registering failure reports not only for compute
task but for other kind of assets (for now, functions which are not
building as part of the execution of a compute task)

- Modifies `ComputeTaskFailureReport`:
    - renamed the model to `AssetFailureReport`
- renamed field `compute_task_key` to `asset_key` (as we can now have a
function key)
    - added field `asset_type` to provide 
- Updates protobuf reflecting the previous changes
- refactor `download_file` in `PermissionMixin` to provide mroe
flexibility (and decouple from DRF)
- create new `FailableTask` (Celery task):
  - centralize the logic to submit asset failure

## How has this been tested?

As this is going to be merged on a branch that is going to be merged to
a POC branch, we use MNIST as a baseline of a working model. We will
deal with failing tests on the POC before merging on main.

## Checklist

- [x] [changelog](../CHANGELOG.md) was updated with notable changes
- [ ] documentation was updated

---------

Signed-off-by: Guilhem Barthes <[email protected]>

backend/substrapp/tasks/tasks_compute_task.py

@@ -16,7 +16,6 @@
 import enum
 import errno
 import os
-from typing import TYPE_CHECKING
 from typing import Any
 
 import celery.exceptions
@@ -25,9 +24,6 @@
 from django.conf import settings
 from rest_framework import status
 
-if TYPE_CHECKING:
-    from billiard.einfo import ExceptionInfo
-
 import orchestrator
 from backend.celery import app
 from substrapp.clients import organization as organization_client

backend/substrapp/tasks/tasks_save_image.py

@@ -3,15 +3,9 @@
 import os
 import pathlib
 from tempfile import TemporaryDirectory
-from typing import TYPE_CHECKING
 from typing import Any
 
 import structlog
-
-if TYPE_CHECKING:
-    from billiard.einfo import ExceptionInfo
-
-from celery import Task
 from django.conf import settings
 from django.core.files import File