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
In this case, you can use the `couch_mgr:db_exists/1` function directly via (no need to wrap it in an anonymous function):

```erlang
FilterFun = fun couch_mgr:db_exists/1,
[{'databases', lists:filter(FilterFun, ...)}].
```