Triage Crawler exceptions: NPM 404s during fetch

[brphelps]

exceptions | summarize Npm404 = countif(outerMessage contains "404 Not Found" and outerMessage contains "npmjsFetch")

[brphelps]

When triaging, make sure to identify whether this issue should or shouldn't be happening -- if it's not truly exceptional, try to convert to a less interesting tracing method (e.g. a warning trace).

[jeffmendoza]

Getting a 404 here: https://github.com/clearlydefined/crawler/blob/master/providers/fetch/npmjsFetch.js#L43

should log and move on, not exception. Based on invalid harvest/package/crawl
Interestingly, we see that in the stacktrace, but then it bubbles up to https://github.com/clearlydefined/crawler/blob/master/ghcrawler/lib/crawler.js#L132 where we see a lot of exceptions come from