changelog: issue an error with ambiguous labels

It is possible to have multiple labels on a pull-request, and thus the following-cases are expected:
- `changelog:skip` and `enhancement` : should the changelog be ignored or should an enhancement be added to the changelog ?
- `bug` and `security`: which one gets selected when multiple values for `type` can be selected in the changelog

For the first case, I have tested it and it seems that the pull-request is just ignored from the changelog, thus making skipping changelog higher priority than mapping in pivot.

For the second case, I haven't tested it, but I think it should also be considered as an error, but this may be already implemented.