Method match() now returns result *and* a reference to list of warnings

If step has a conition based on var but var has not been
specified in params, it is not possible to decide if this
condition is fulfilled.

This case cannot be copletely avoided, byu now it will
generate a useful warning.

Functional behavor is that such steps are *allowed* and
eventually revealed to tester.  (Possibly in a helpful manner.)