Ensure api errors remain strings + provide structural hints (#27563)

* - ensure errors are strings + provide strucrual hints

Instead of just overwriting the (string) erorr messages, swap in the
malli string description at [:errors :<field-name>] key. Adds humanized
error (which highlights which parts of req were not sufficient) under
the :specific-errors key.

* fix tests + note about auto-cohersion