Change the hash checker to not respond with a !200 error code

[peterbe]

At the moment the hash check in the collector app responds with 200 OK if the hash is already known and 404 Not Found if it's not been collected before.

Every time I see those 404 errors I wonder if there's something wrong. Like, if it's a bug.
In the Web Console the 404 AJAX requests stands out in an ugly way too.

I think we should use 200 OK for both and use the content to say Yes or No.