`t:progressiveText` should tolerate 5xx HTTP errors (#7614)

* `t:progressiveText` should tolerate 5xx HTTP errors

* Applying @daniel-beck’s suggestion from https://github.com/jenkinsci/jenkins/pull/7614#issuecomment-1410378581

* Better handle a 404 https://github.com/jenkinsci/jenkins/pull/7614#issuecomment-1411755404



* Restoring trailing slash in `fallbackUrl` to match original behavior, lest there be another redirect

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Co-authored-by: Daniel Beck <1831569+daniel-beck@users.noreply.github.com>