Ensure required settings are defined correctly.
Ensure required settings are defined correctly.

Possible fix for #424. If a setting has not been overidden in a subclass
or set in the user's config then raise an exception.

enkore/i3pystatus,ncoop/i3pystatus,fmarchenko/i3pystatus,richese/i3pystatus,drwahl/i3pystatus,Arvedui/i3pystatus,facetoe/i3pystatus,Arvedui/i3pystatus,richese/i3pystatus,fmarchenko/i3pystatus,teto/i3pystatus,teto/i3pystatus,enkore/i3pystatus,yang-ling/i3pystatus,drwahl/i3pystatus,schroeji/i3pystatus,facetoe/i3pystatus,ncoop/i3pystatus,m45t3r/i3pystatus,schroeji/i3pystatus,yang-ling/i3pystatus,m45t3r/i3pystatus