make indexing gateway SOAP interface not throw ClientException upon missing document

External indexers might consider SOAP exceptions as application failure. However users can delete documents while the database is being reindexed. In order to stop throwing ClientExceptions we might want to change the WSIndexingGatewayBean implementation to return empty descriptions for missing documents.

An explicit API marker for missing document would be even better but that would impact the WSDL definitions hence force external plug implemenation to get upgraded too.