[BUG] Improve error response codes.

[vamsi-amazon]

Currently all the internal errors throw Service Unavailable exception which is not apt and should throw internal server failure instead.
Code Pointer: https://github.com/opensearch-project/sql/blob/main/plugin/src/main/java/org/opensearch/sql/plugin/rest/RestPPLQueryAction.java#L148

The ask of this issue is to fix these error response codes in both sql and ppl APIs.

[penghuo]

• Internal Server error should return 500.
• Customer error should return 4xx.