Address CompoundRetrieverBuilder Failure Handling

[john-wagster]

exploring options for improving error handling for CompoundRetrieverBuilder particularly in the case where a response of sub retriever is 2xx but some shard failed to retrieve data.

addresses: #136529

[john-wagster]

@pmpailis curious if this is roughly what you were thinking in terms of improving error handling and if you had other thoughts about how this should behave or additional checks. What else would be nice to have here? I thought about enhancing the error message handling within the if (false == failures.isEmpty()) { block to include a list of the failure messages as well. How do you feel about that?

[pmpailis]

Thanks @john-wagster for picking this up! ❤️ Yeah, this is pretty much what I was thinking as well; only minor comments on the type of exception that we would throw (to avoid constantly 5xx).

Will take a look at adding a test case as well.

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Hi @john-wagster, I've created a changelog YAML for you.

[john-wagster]

@pmpailis I created a mock test which helped with cleaning up the code. Let me know what you think about that and the current state of the code. I also realized i have no idea what version labels this should actually be applied to so I just put them all on here. I think that's everything that's currently supported??? Thoughts on what releases this should target would be welcome.