Show the stage name (if one is set) when failing the stage in the
validator. This closes#1007, example output:
```
€ python3 -m osbuild supakeen-os.json
supakeen-os.json has errors:
pipelines[0].stages[0]
could not find schema information for 'org.osbuild.rpmb'
.pipelines[0].stages[0].inputs.packages:
could not find schema information for 'org.osbuild.filesz'
```