api/serializers: Handle case where an exception can not be rebuilt

It exists cases where a custom exception type cannot be rebuilt from its serialized form, see https://sentry.softwareheritage.org/organizations/swh/issues/4959/?query=is%3Aunresolved

As a fallback, build a base Exception with the message of the serialized exception.

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Migrated from D4730 (view on Phabricator)