Examine the behavior of `allow_partial_results` with ES|QL for 500 status codes

[quackaplop]

Currently ES|QL queries that fail with 500s (generally indicating bugs and internal errors) will return 200 when allow_partial_results is set to true. This can be considered a correctness issue for the user - we are not masking missing indices or failed shards, rather our own bugs.
This issue tracks:

• Confirming that this is what actually happens
• Deciding how to mitigate this - we need a mechanism to make the user aware that something went seriously wrong (which 500 would indicate)

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)