feat: print warning when lambda cannot be resolved

**Description**

I just mistyped the `snakeCase` build-in lambda by using `{{variable.snake_case()}}` instead of `{{variable.snakeCase()}}` causing the output of the lambda to be just an empty string.

This was obviously my mistake, but I think it would be great if the tool prints at least a warning that the used lambda could not be resolved. This makes building bricks and searching for errors easier.

**Requirements**

- detect unresolvable lambdas
- print an error messge

**Additional Context**

Let me know if you need additional information.

Thanks for the great tool 🧱. It's so easy to do code generation 👍🏻