Issues with catchError and NOT_BUILT

After #87 things are working much better but I do have some code that will do something like this:

```
catchError(buildResult: null, stageResult: 'NOT_BUILT') {
echo 'Skipping static analysis'

sh 'exit 1'
}
```

This results in the stage being stuck in a `Pending` state in GitHub. Personally, I think these should get considered as successful or I guess just not considered at all when determining stage state.