Mark redirect URLs as 'seen'; add to TTLHashSet

Mark URLs returning a HTTP 3XX redirect status as 'seen' in the
Redis-backed TTLHashSet so that we don't try crawling them again until
the TTLHashSet expiry time (currently 48 hours) has passed.

Also, change the if block to use `switch` for readability.
Change INFO log message to ERROR
Merge pull request #59 from alphagov/76585878-mark-redirect-urls-as-seen

[#76585878] Mark redirect urls as seen
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
Move log line and rephrase message

Move the log line down to where we start crawling the URL and rephrase
the log message to make it clearer what's happening.