Internal: expose the indices names every action relates to if applicable

If a request relates to indices, expose which ones it relates to in a generic manner.
good point, `IllegalStateException` makes more sense. I wouldn't use an assert as this can happen whenever one calls `requestedIndices` without having set a search request before. This is also related to whether we should rely on a validated request or not, since `validate` throws error when a search request is null. I think we should keep the exception and make it illegal state.