better error in the case where init is called on an existing db

the reason I shorted it is because the full error when realized is
```
E1215 21:09:35.763301   62288 server.go:160] Failed to bootstrap cluster: store.go:445: non-empty engine {[ssd] }=/tmp/db (first key: "%\x80\x80\x80\x86ˑ\xca\xee\x00")
```
So the previous error was essentially saying "failed to bootstrap cluster" twice in the output.