Error message to explain that non differentiable arguments should be marked as DoesNotExist(), not Zero()

[mzgubic]

Currently, if someone mistakenly uses Zero() instead of DoesNotExist() inside the rrule

function partly_dne(a, i)
    return a[i]
end

function ChainRulesCore.rrule(::typeof(partly_dne), a, i)
    y = partly_dne(a, i)
    function partly_dne_pullback(ȳ)
        grad = zeros(size(a))
        grad[i] = ȳ
        return (NO_FIELDS, grad, Zero())
    end
    return y, partly_dne_pullback
end

The error message isn't very clear

julia> rrule_test(partly_dne, rand(), (rand(4), rand(4)), (1, nothing))
Test Failed at /Users/mzgubic/Projects/ChainRules.jl/dev/ChainRulesTestUtils/src/testers.jl:295
  Expression: x̄_ad isa DoesNotExist
   Evaluated: Zero() isa DoesNotExist
ERROR: There was an error during testing