Nominatim: reject suggest() per usage policy

The Nominatim usage policy forbids auto-complete: "you must not implement
such a service on the client side using the API". Implement suggest() so
that it throws a dedicated SuggestUnsupportedError referencing the policy,
rather than leaving the method absent.

Since the control feature-detects suggest() by its presence, it now catches
SuggestUnsupportedError, detaches the input listener and stops issuing
suggestion requests -- so the default geocoder keeps working on submit.

https://operations.osmfoundation.org/policies/nominatim/