Return 429 on ES API when no shards available

[rdettai]

Description

Closes #5565

How was this PR tested?

N/A

[rdettai]

429 is required for the JS client to retry, but note that in the Ingest (V2) API the response code is 503 Unavailable

quickwit/quickwit/quickwit-ingest/src/error.rs

Lines 116 to 118 in 2f90a0c

	IngestFailureReason::NoShardsAvailable => {
	IngestServiceError::Unavailable("no shards available".to_string())
	}