friendly error when using rule instead of state

[CarloLucibello]

trafficstars

[mcabbott]

I like friendly errors, but I think this is going to produce ambiguities with Flux, method 7:

julia> using Flux, Optimisers

julia> methods(Optimisers.update!)
# 8 methods for generic function "update!" from Optimisers:
 [1] update!(x::AbstractArray, x̄)
     @ Flux ~/.julia/packages/Flux/HBF2N/src/deprecations.jl:21
 [2] update!(opt::Flux.Optimise.AbstractOptimiser, ::Params, grads::Union{Tuple, NamedTuple})
     @ Flux ~/.julia/packages/Flux/HBF2N/src/deprecations.jl:175
 [3] update!(opt::Flux.Optimise.AbstractOptimiser, x::AbstractArray, x̄)
     @ Flux.Optimise ~/.julia/packages/Flux/HBF2N/src/optimise/train.jl:22
 [4] update!(opt::Flux.Optimise.AbstractOptimiser, xs::Params, gs)
     @ Flux.Optimise ~/.julia/packages/Flux/HBF2N/src/optimise/train.jl:28
 [5] update!(opt::Flux.Optimise.AbstractOptimiser, model::Chain, grads::Tuple)
     @ Flux ~/.julia/packages/Flux/HBF2N/src/deprecations.jl:169
 [6] update!(opt::Flux.Optimise.AbstractOptimiser, model, grad)
     @ Flux ~/.julia/packages/Flux/HBF2N/src/deprecations.jl:149
 [7] update!(opt, model::Chain, grads::Tuple)
     @ Flux ~/.julia/packages/Flux/HBF2N/src/deprecations.jl:162
 [8] update!(tree, model, grad, higher...)
     @ ~/.julia/packages/Optimisers/yDIWk/src/interface.jl:70

I guess we can fix that in Flux, but should not forget.