graph: add Is*(err error) checker tests
graph: add Is*(err error) checker tests

barakmich/cayley,pombredanne/cayley,kortschak/cayley,h4ck3rm1k3/cayley,cayleygraph/cayley,deskr/cayley,cayleygraph/cayley,barakmich/cayley,kortschak/cayley,pombredanne/cayley,kortschak/cayley,barakmich/cayley,h4ck3rm1k3/cayley,h4ck3rm1k3/cayley,kortschak/cayley,barakmich/cayley,h4ck3rm1k3/cayley,deskr/cayley,deskr/cayley,deskr/cayley,h4ck3rm1k3/cayley,deskr/cayley,pombredanne/cayley,pombredanne/cayley,barakmich/cayley,kortschak/cayley