Review validation errors thrown

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 ? * * *"
```

Start by locating where Superstruct validation errors are caught and formatted, then compare the current output with the example in the issue. Done means the error messages consistently expose useful validation details and relevant tests cover the improved wording.

Written by the indexing model from the issue text.