Query-schedulers returns error which can't be parsed by query-frontend, resulting in 5xx

[replay]

Describe the bug

The error which gets returned when the query scheduler has too many outstanding requests is a simple string, here:

mimir/pkg/frontend/v2/frontend_scheduler_worker.go

Lines 334 to 337 in 9e7f5a2

	HttpResponse: &httpgrpc.HTTPResponse{
	Code: http.StatusTooManyRequests,
	Body: []byte("too many outstanding requests"),
	},

When the query-frontend receives that error it tries to unmarshal it as JSON, which doesn't work, here:

mimir/pkg/frontend/querymiddleware/codec.go

Line 287 in 3de8a38

if err := json.Unmarshal(buf, &resp); err != nil {

This results in a 5xx error and log message like this one:

[replay]

This is duplicate with #2447