Internal: streamline use of IndexClosedException

[javanna]

With #6475 we introduced the use of a new IndexClosedException that gets thrown whenever we refer to closed indices within apis that don't allow them (behaviour can also be changed through indices options in some cases). We should streamline the use of this exception and use it within single index apis like index api, which currently return an index block during the execution while we could throw the newly added IndexClosedException instead.

[clintongormley]

@javanna I've labelled this as adoptme, unless you're planning on doing it? if so, could you assign it to yourself please?

[javanna]

Working on it @clintongormley ;)

[javanna]

Marking this as breaking as this change affects what we return when using one of the following apis against closed indices: analyze, bulk, index, update, delete, explain, get, multi_get, mlt, term vector, multi_term vector. We now return:

{"error":"IndexClosedException[[test] closed]","status":403}

instead of

{"error":"ClusterBlockException[blocked by: [FORBIDDEN/4/index closed];]","status":403}