fix: clarify error message for JavaScript evaluation failure

[Sana9058]

Summary

Improves the error message thrown in evaluateJavaScript when required
inputs are missing.

What changed

• Replaced a generic error message that printed raw fn and data
• Added a clearer, user-friendly message explaining that template data
  or function source is missing

Motivation

The previous error message could expose large objects in logs and was
hard to understand during debugging. This change makes the failure
reason explicit without changing any behavior.

Notes

• No functional or logic changes
• Error condition remains the same

[Sana9058]

Thanks for taking a look!

This change only improves the clarity of the error message when JavaScript
evaluation is attempted without required inputs. No behavior or logic was
changed.

Happy to adjust or split commits if you’d prefer a different structure.

[mttrbrts]

Unfortunately we don't accept documentation changes from first contributors. Thanks for your contribution