s3api: answer throttling rejections as SlowDown

ErrTooManyRequest and ErrRequestBytesExceed reported made-up codes;
AWS serves these throttling rejections as SlowDown.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/s3api/s3err/s3api_errors.go

@@ -627,12 +627,12 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		HTTPStatusCode: http.StatusConflict,
 	},
 	ErrTooManyRequest: {
-		Code:           "ErrTooManyRequest",
+		Code:           "SlowDown",
 		Description:    "Too many simultaneous request count",
 		HTTPStatusCode: http.StatusServiceUnavailable,
 	},
 	ErrRequestBytesExceed: {
-		Code:           "ErrRequestBytesExceed",
+		Code:           "SlowDown",
 		Description:    "Simultaneous request bytes exceed limitations",
 		HTTPStatusCode: http.StatusServiceUnavailable,
 	},