Snapshot root directory missing while browsing

Sentry Issue: SWH-WEBAPP-5W6

AssertionError: 
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django_ratelimit/decorators.py", line 27, in _wrapped
    return fn(request, *args, **kw)
  File "/opt/swh/.local/lib/python3.10/site-packages/swh/web/browse/views/content.py", line 267, in content_display
    content_data = _get_content_from_request(request)
  File "/opt/swh/.local/lib/python3.10/site-packages/swh/web/browse/views/content.py", line 244, in _get_content_from_request
    assert root_directory is not None  # to keep mypy happy

The issue only happens on staging and is due to a missing revision in staging storage (see https://webapp.staging.swh.network/browse/origin/directory/?branch=refs/heads/20200428-block-fixes&origin_url=https://git.kernel.org/pub/scm/linux/kernel/git/mcgrof/linux-next.git&snapshot=4c92b5911fe0ec0ac0f3f1064c50c783d4f89b1d).

While this type of issue should not happen in production, we should check revision/release existence when resolving a snapshot context and return a 404 error with detailed error message instead of a 500 error.

thanks for the clarification, I made !1369 (merged) to raise a NotFoundExc istead of assert-ing the existence of a root directory

mentioned in merge request !1369 (merged)

closed with merge request !1369 (merged)