Allow "connection refused" errors from helper

Very occasionally the helper test will fail with "connection refused"
instead of "EOF". As seen here:

https://ci-new.alphagov.co.uk/job/govuk_infrastructure_cdn-acceptance-test_cloudflare/15/console

I suspect this happens when the connection is reset during the intial TCP
setup (dial) rather than in the middle of a request. This is similar to:

https://github.com/alphagov/govuk_crawler_worker/commit/fdde4d1723866be44e4fc054765024a120f904df#diff-2cad8ba8c8780d07a5167b083d6a0cb8R62

So match both errors. But without the complexity of a type assertion.
Merge pull request #89 from alphagov/helper_cnx_refused

Allow "connection refused" errors from helper