Handle duplicate assessment_item_ids with a semantic error rather than a 500

[sentry-io]

IntegrityError: duplicate key value violates unique constraint "assessmentitem_unique_keypair_constraint"

Sentry Issue: STUDIO-H5X

UniqueViolation: duplicate key value violates unique constraint "assessmentitem_unique_keypair_constraint"
DETAIL:  Key (assessment_id, contentnode_id)=(7cec5924685552bc854f5e3df69aa65d, d4b9932897d34ac5b140917c6a05f224) already exists.

  File "django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "newrelic/hooks/database_psycopg2.py", line 64, in execute
    return super(CursorWrapper, self).execute(sql, parameters, *args,
  File "newrelic/hooks/database_dbapi2.py", line 38, in execute
    return self.__wrapped__.execute(sql, parameters,
  File "django_prometheus/db/common.py", line 71, in execute
    return super().execute(*args, **kwargs)

IntegrityError: duplicate key value violates unique constraint "assessmentitem_unique_keypair_constraint"
DETAIL:  Key (assessment_id, contentnode_id)=(7cec5924685552bc854f5e3df69aa65d, d4b9932897d34ac5b140917c6a05f224) already exists.

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  File "contentcuration/views/internal.py", line 298, in api_add_nodes_to_tree
    "root_ids": convert_data_to_nodes(request.user, content_data, parent_id)
  File "contentcuration/views/internal.py", line 666, in convert_data_to_nodes
    create_exercises(user, new_node, node_data["questions"])
  File "contentcuration/views/internal.py", line 797, in create_exercises
    question_obj.save()