"swh scheduler task add" command should print an error message in case of unknown task

When one tries to schedule a not registered task using swh-scheduler CLI, the following traceback is displayed in command output:

14:51 $ doco exec swh-scheduler swh scheduler task add -p oneshot list-foo
Traceback (most recent call last):
  File "/srv/softwareheritage/venv/bin/swh", line 8, in <module>
    sys.exit(main())
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/swh/core/cli/__init__.py", line 184, in main
    return swh(auto_envvar_prefix="SWH")
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/swh/scheduler/cli/task.py", line 284, in schedule_task
    created = scheduler.create_tasks([task])
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/swh/core/db/common.py", line 73, in _meth
    return meth(self, *args, db=db, cur=cur, **kwargs)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/swh/scheduler/backend.py", line 605, in create_tasks
    cur.execute(query)
  File "/srv/softwareheritage/venv/lib/python3.7/site-packages/psycopg2/extras.py", line 251, in execute
    return super(RealDictCursor, self).execute(query, vars)
psycopg2.errors.NotNullViolation: null value in column "retries_left" violates not-null constraint
DETAIL:  Failing row contains (1, list-foo, {"args": [], "kwargs": {}}, 2022-09-14 12:52:05.061432+00, null, next_run_not_scheduled, oneshot, null, null).
CONTEXT:  SQL statement "insert into task (type, arguments, next_run, status, current_interval, policy,
                    retries_left, priority)
    select type, arguments, next_run, status, current_interval, policy,
           retries_left, priority
    from tmp_task t
    where not exists(select 1
                     from task
                     where type = t.type and
                           md5(arguments::text) = md5(t.arguments::text) and
                           arguments = t.arguments and
                           policy = t.policy and
                           priority is not distinct from t.priority and
                           status = t.status)"
PL/pgSQL function swh_scheduler_create_tasks_from_temp() line 12 at SQL statement

As this is not really user friendly, an error message should be displayed instead, something like Error: no celery task named <task_name> is registered in swh-scheduler database.

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Migrated from T4535 (view on Phabricator)