failure member in ShardSearchFailure is never serialized

[rjernst]

The Throwable failure member of ShardSearchFailure is taken on construction, but only used to generate a detailed error message to place in the reason member. It is never serialized/deserialized. But there is a failure() function that returns the Throwable. In the case of a transported failure, it is always null.

I think we should just remove failure (it is only set in one of the many constructors), and in the two tests that use it, use reason instead.