Today we have a intermediate hierarchy for shard and index exceptions
which makes it hard to introduce generic exceptions like ResourceNotFoundException
intoduced in this commit. This commit breaks up the hierarchy by adding index and shard
as a special internal header that gets rendered for every exception that fills that header.
This commit removes dedicated exceptions like `IndexMissingException` or
`IndexShardMissingException` in favour of `ResourceNotFoundException`
7db293c616