[#76585878] Keep errors separate

The `nil` value returned by `*TTLHashSet.Set` as an `error` type was
overwriting the `err` variable that stores the error returned by
`CrawlURL()`, which led to a nil errors being logged such as:

    WARN[0000] Couldn't crawl (rejecting): http://127.0.0.1:51892 <nil>

This also means we can use the `fallthrough` keyword to reduce duplication.