sbuild workflow has an unclear error when there are no environments

From #829 (closed):

@paravoid created an upload-to-experimental workflow that failed to orchestrate sbuild with:

Orchestrator failed: None of the workflow architectures are supported by this package: workflow: all package: any

The cause here is that we have no environments for debian:experimental in our environments collection. This could have a much clearer error.

If get_available_architectures returns nothing, we should record an error that states this.