all: use errors.As for inspecting errors that implement httpserver.ErrorWithStatusCode

lib/httpserver/httpserver.go

@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"context"
 	"crypto/tls"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -487,8 +488,9 @@ func Errorf(w http.ResponseWriter, format string, args ...interface{}) {
 
 	// Extract statusCode from args
 	statusCode := http.StatusBadRequest
+	var esc *ErrorWithStatusCode
 	for _, arg := range args {
-		if esc, ok := arg.(*ErrorWithStatusCode); ok {
+		if err, ok := arg.(error); ok && errors.As(err, &esc) {
 			statusCode = esc.StatusCode
 			break
 		}