Add method to issue messages during config phase

[larrybarello]

Describe the enhancement

I use command line key/value to control yaml configuration. There is no mechanism to reflect to the console when a valid parameter is missing

Use cases

Substitute:
  Do_this: some_path/some_file.yaml
  Do_that: ...
Packages:
  F0: ${" error message" if some_key undefined else {}}
  F1: ${do_this if some_key == "foo" else { }}
  F2: ${Do_that if some_key == "bar" else {"print an error message }}


### Visual examples

_No response_

### Anything else?

_No response_