Implement Stringer interface for DeviceState (#49)

* Implement `Stringer` interface for `DeviceState`

If device adoption fails the error message isn't very readable at the moment.

```
Error: unexpected state '10', wanted target '2'.
```

* Use `stringer`

.github/workflows/ci.yaml

@@ -42,6 +42,8 @@ jobs:
       - uses: "actions/setup-go@v2"
         with:
           go-version: "${{ env.GO_VERSION }}"
+      - run: "go get golang.org/x/tools/cmd/stringer"
+
       - run: "go generate ./..."
       - run: "git diff --compact-summary --exit-code"