Better warning if po(<id>) does not work, but ppl(<id>) exists

[sebffischer]

Inspired by this SO post: https://stackoverflow.com/users/17945841/programming-noob

[mb706]

Link is appears broken to me.

Since our dictionaries all inherit from the same base class, we could also think of having a central registry of all dictionaries in mlr3misc, where did_you_mean looks for similar names. So if someone wants a pipeop but uses lrn(), or wants a filter but uses po() etc., one would get a fitting hint.