Fix regressions with failOnFailedSeed option

[tw4l]

Fixes #563

This PR makes a few changes to fix a regression in behavior around failOnFailedSeed for the 1.x releases:

• Fail with exit code 1, not 17, when pages are unreachable due to DNS not resolving or other network errors if the page is a seed and failOnFailedSeed is set
• Extend tests, add test to ensure crawl succeeds on 404 seed status code if failOnINvalidStatus isn't set

(Edited after reverting change related to invalid statuses)

[ikreymer]

• Fail crawl if seed has 0/4xx/5xx status code, even if failOnInvalidStatus is not set (otherwise, users who previously only had to pass one argument now have to pass two, which is a breaking change likely to result in unexpected behavior; for non-seeds, we only consider pages with a 4xx/5xx status code a failure if failOnInvalidStatus is set)

Hmm, I think that may be more confusing. The --failOnInvalidStatus was introduced in 1.x, and yes it is a breaking change from 0.x, but I think that's ok, given it was a major version change. I think there should be a way to only fail on invalid/unloadable pages, and pages that do load, but return an error code. The browser does not treat 4xx/5xx responses as failures, as they do load, and #563 is referring to invalid/unloadable URLs specifically it seems.

[tw4l]

@ikreymer Reverted and tests extended to include status code, which let me catch another issue in the argParser. Should be all good now!

[ikreymer]

Thanks for the cli updates to make it more clear