graph: fix Is*(err error) checkers to properly compare against *DeltaError
graph: fix Is*(err error) checkers to properly compare against *DeltaError

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