s3api: Better handle 498/429 responses
Currently, they just 500 as an unexpected response status. Much better
would be S3's '503 Slow Down' response.
Of course, that's all dependent on where you place ratelimit in your
pipeline -- and we haven't really given clear guidance on that. I'm not
actually sure you *want* ratelimit to be after s3api and auth... but if
you *do*, let's at least handle it gracefully.
(cherry picked from commit f33c061ae92f33ea467c58749d3f15d2b1cc942c)