KAZOO-3115 cb_faxes GET incoming can't be validated if one of modbs doesn't exist

If freshly created account tries to list incoming faxes it receives an error because crossbar tries to look at previous month modb which doesn't exists. 

Nonexistent DBs should be filtered from cb_faxes:incoming_summary
i think this should be checked in crossbar_doc:load_view so that it handles other possible pvt_types