Build sometimes breaks with a stringer complaint

```make
(which stringer > /dev/null) || go get golang.org/x/tools/cmd/stringer
(which jsonenums > /dev/null) || go get github.com/campoy/jsonenums
cd core && go generate && go test
stringer: no values defined for type StopReason
step.go:69: running "stringer": exit status 1
Makefile:10: recipe for target 'test' failed
make: *** [test] Error 1
```