fix mistake in non-tuple output message

[oxinabox]

∂s was renamed to ad_cotangents, but we missed it in this error message.
So I fixed that and I also changed it to use test_msg rather than erroring.
added a test for this case, making sure it failed rather than erroring.

I also changed the length one after to not define a variable called msg since that was confusing as to which was meant.

[codecov-commenter]

Codecov Report

Merging #177 (68fc041) into master (e21bf34) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #177      +/-   ##
==========================================
- Coverage   87.45%   87.41%   -0.05%     
==========================================
  Files          10       10              
  Lines         295      294       -1     
==========================================
- Hits          258      257       -1     
  Misses         37       37              

Impacted Files	Coverage Δ
src/testers.jl	93.69% <100.00%> (-0.06%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e21bf34...68fc041. Read the comment docs.

[mzgubic]

Thanks, LGTM, just needs fixing the docs

[github-actions]

[Documenter (fix doctests)] _{reported by reviewdog 🐶}

Suggested change

	Test Summary: | Pass Total
	test_frule: two2three on Float64,Float64 | 6 6

[github-actions]

[Documenter (fix doctests)] _{reported by reviewdog 🐶}

Suggested change

	Test Summary: | Pass Total
	test_rrule: two2three on Float64,Float64 | 7 7
	test_rrule: two2three on Float64,Float64 | 8 8

[github-actions]

[Documenter (fix doctests)] _{reported by reviewdog 🐶}

Suggested change

	Test Summary: | Pass Total
	test_scalar: relu at 0.5 | 9 9
	test_scalar: relu at 0.5 | 10 10

[github-actions]

[Documenter (fix doctests)] _{reported by reviewdog 🐶}

Suggested change

	Test Summary: | Pass Total
	test_scalar: relu at -0.5 | 9 9
	test_scalar: relu at -0.5 | 10 10

[oxinabox]

the auto suggesting of fixes seems to be be doing a bad.
I am tempted to leave it in and see if it comes up good after: it might be some how confused since I have messed with documenter so much.