add error message for integers that represent reals

e.g. example by @SomTambe

julia> f(x) = 8x + 10*sin(x)
f (generic function with 1 method)
julia> function ChainRulesCore.rrule(::typeof(f), x)
           y = f(x)
           function ∇f(Δ)
               ∇ = 8 + 10*cos(x)
               @show ∇ Δ
               return ChainRulesCore.NoTangent(), Δ * ∇
           end
           return y, ∇f
       end
julia> test_rrule(f, 2);
∇ = 3.838531634528576
Δ = 2.31
∇ = 3.838531634528576
Δ = 2.31
test_rrule: f on Int64: Test Failed at /home/somvt/.julia/packages/ChainRulesTestUtils/AX7fv/src/testers.jl:278
  Expression: ad_cotangent isa NoTangent
   Evaluated: 8.86700807576101 isa NoTangent
...
Test Summary:          | Pass  Fail  Total
test_rrule: f on Int64 |    4     1      5
ERROR: Some tests did not pass: 4 passed, 1 failed, 0 errored, 0 broken.

the error message is confusing, we could special case it for actual::AbstractFloat, expected::NoTangent as suggested by @oxinabox

Reproduce the shown failure with test_rrule(f, 2) and start at the tester entry point reported as testers.jl:278. Review the actual::AbstractFloat and expected::NoTangent case suggested in the issue, then verify that the resulting failure message clearly explains the integer-input mismatch.