Review validation errors thrown

[FrederikBolding]

Sometimes the errors thrown by superstruct are hard to understand because they don't include any details from the validation itself.

We should review the error messages we throw and try to improve them.

Example:

Failed to fetch Snap "local:http://localhost:8080/": At path: initialPermissions.endowment:cronjob.jobs.0.expression -- Expected a value of type string, but received: "0 0/1 0 ? * * *"