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>
Use fallthrough keyword to reduce duplication

See: http://golang.org/ref/spec#Fallthrough_statements
Merge pull request #60 from alphagov/76585878-mark-redirect-urls-as-seen

[#76585878] Keep errors separate