Current DefaultRestClient impl has several indirections from invoke methods to one another, up to a chain of delegation involving 2 public methods + 2 private methods, all responsible for various aspects of building the invocation (invocation itself, parameterization, error-handling).

Proposing to address that while expanding RestClient's invoke capabilities to take (optionally) an error-handler, passed from the consumer (knowing best what to do then).

At the same time, we reckon #invoke method accepting whole request body as argument needs to go, in favor of declared bodies in rest calls, and support for template values there.

restfn may as well accept an error-handler.