`when` Function with Incorrect Name Triggers Non-Descriptive Error Message

[yegor256]

This is what I'm doing:

$ cat simple.yml
name: foo
pattern: ⟦ !B ⟧
result: ⟦ !B ⟧
when:
  wrong-name:

$ cat foo.phi
{⟦ k ↦ "foo" ⟧}

$ phino rewrite --rule simple.yml --sweet --max-depth=2 < foo.phi
[ERROR]: Aeson exception:
Error in $.when: empty

I suggest we print something more meaningful, like The function "wrong-name" is not found.