Don't throw an exception when the referenced FK table is not visible

As retrieving meta data for tables can be quite expensive (regarding the time need to do this on an Oracle database) I decided to restrict it by extending the `DefaultDatabaseTableFilter`. I found that this works quite well until `DatabaseMetaDataInspectorTool` tries to read information from foreign keys that are no longer visible. 

This patch takes care of this.

Please have a look an merge it - or comment if you want something to be changed.

Thanks, Alexander.