Check if scenario file exists before attempting to load

[koen-vg]

A very simple quality of life improvement. Useful when running the create_scenarios rule the first time.

When wishing to generate a new scenarios files (say, "scenarios2.yaml"), one could change the corresponding config option (run/scenarios/file) and run the create_scenarios rule. However, if say "scenarios2.yaml" doesn't exist yet, the get_scenarios function fails. An awkward solution is to first create "scenarios2.yaml" manually (and put in some mock scenarios, otherwise scenarios.keys() fails!), and then run create_scenarios. With this check, that's not necessary anymore.

Checklist

• I tested my contribution locally and it seems to work fine.
• Code and workflow changes are sufficiently documented.
• Changed dependencies are added to envs/environment.yaml.
• Changes in configuration options are added in all of config.default.yaml.
• Changes in configuration options are also documented in doc/configtables/*.csv.
• A release note doc/release_notes.rst is added.

[FabianHofmann]

makes sense!