[REST] Missing scroll id now returns 404

A bad/non-existing scroll ID used to return a 200, however a 404 might be more useful.
Also, this PR returns the right Exception (SearchContextMissingException) in the Java API.

TODO: I need some more feedback for this PR. I think this solution is not optimal and there might be a better option (checking the exception type twice is not what I want to do, I do this in order to return a real exception instead of a shard failure, but it is not a good solution codewise and I think there are better solutions which I do not see at the moment).

Closes #5729