[Factory] Improve logging when hook validation fails

This PR improves the log message when a hook validation fails... It will ease the next bugfixes when we read logs :)

Before: 
```
Github Hook - Invalid Request - The github hook request is not valid.
```

After: 
```
Github Hook - Invalid Request - The github hook request is not valid (The child node "before" at path "hook" must be configured).
```

I have added it as it seems the provider fails when we delete a branch...

ping @tmartin :kiss: