Warn the user if the scenario.automated.yaml is outdated or non existent

[lindnemi]

The goal of this PR is to warn the user about some errors that might occur when using the scenario management in pypsa-de incorrectly

Ideally the workflow wouldn't just print a warning, but fail but i did not find out how to do that yet (because it should only fail when ariadne_all is called and not when build_scenarios is called)

Even more ideally, build_scenarios would be included in the default workflow, but that seems even more complicated to achieve as that means modifying the snakemake config in runtime.

Before asking for a review for this PR make sure to complete the following checklist:

• Workflow with target rule ariadne_all completes without errors
• The logic of export_ariadne_variables has been adapted to the changes
• One or several figures that validate the changes in the PR have been posted as a comment
• A brief description of the changes has been added to Changelog.md
• The latest main has been merged into the PR
• The config has a new prefix of the format YYYYMMDDdescriptive_title