Further restrict / improve Difference methods

[fingolfin]

(Taken from gap-system/gap#2164)

Consider these two inputs:

Difference(Rationals, [1/2]);
R:=GroupRing(Integers, FreeGroup(1));; Difference(R, [R.1]);

Without resclasses, one gets this:

gap> Difference(Rationals, [1/2]);
Error, sorry, cannot subtract from the infinite domain <D> at GAPROOT/lib/coll.gi:3002 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:1
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>

gap> R:=GroupRing(Integers, FreeGroup(1));; Difference(R, [R.1]);
Error, sorry, cannot subtract from the infinite domain <D> at GAPROOT/lib/coll.gi:3002 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:1
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk>

With resclasses, one also gets errors, but less helpful ones:

gap> Difference(Rationals, [1/2]);
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 2nd choice method found for `ResidueClassUnionCons' on 6 arguments at GAPROOT/lib/methsel2.g:249 called from
ResidueClassUnionCons( IsResidueClassUnion, R, m, r, included, excluded ) at GAPROOT/pkg/resclasses/lib/resclass.gi:886 called from
ResidueClassUnionNC( R, One( R ), [ Zero( R ) ], [  ], Set( S ) ) at GAPROOT/pkg/resclasses/lib/resclass.gi:2126 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:1
type 'quit;' to quit to outer loop
brk>

and similar for the second input.

It would be nice if the better error message was retained, e.g. by further restricting the inputs to which resclasses' Difference method applies.