Maestro should notify repo owners if a subscription can't be executed due to a user error

### Context

Recently we've seen some failed WorkItems in our logs that we're the result of incorrect repo configuration, not the fault of the service.
When this happens, ideally we wouldn't get a failed workitem, but the repo owners would be notified of it

### Failure types
- Incorrect inclusion/exclusion filters in source-mappings: https://github.com/dotnet/arcade-services/issues/6053
- Git file with mixed line endings, always appearing dirty: https://github.com/dotnet/arcade-services/issues/6052
- Missing versioning files in the repo (eg: no global.json or Version.Details.xml)
- Subscription not updatable due to merge conflict in the existing PR itself (happens in version.details.xml)
- Subscription target branch doesn't exist