Mimir returns HTTP status 499 instead of 5xx

[jkirkwood]

We've been receiving several 499 HTTP responses from Mimir when 5xx response code seems more appropriate. This prevents many HTTP clients from automatically retrying the request if enabled.

{"status":"error","errorType":"canceled","error":"expanding series: grpc: the client connection is closing"}

{"status":"error","errorType":"canceled","error":"attempted to read series at index 0 from store-gateway chunks stream, but the stream has failed: grpc: the client connection is closing"}