Display HTTP errors from server

Upstreaming change from golang/go@39366326.

HTTP body will be read and displayed when both conditions are true:
* Response includes non-empty header "X-Go-Pprof".
* Content-Type is text/plain.

internal/driver/fetch.go

 	"bytes"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"net/http"
 	"net/url"
 	"os"
 	"os/exec"
 	"path/filepath"
 	"strconv"
+	"strings"
 	"sync"
 	"time"
 
 		return nil, fmt.Errorf("http fetch: %v", err)
 	}
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("server response: %s", resp.Status)
+		defer resp.Body.Close()
+		return nil, statusCodeError(resp)
 	}
 
 	return resp.Body, nil
 }
 
+func statusCodeError(resp *http.Response) error {
+	if resp.Header.Get("X-Go-Pprof") != "" && strings.Contains(resp.Header.Get("Content-Type"), "text/plain") {
+		// error is from pprof endpoint
+		body, err := ioutil.ReadAll(resp.Body)
+		if err == nil {
+			return fmt.Errorf("server response: %s - %s", resp.Status, body)
+		}
+	}
+	return fmt.Errorf("server response: %s", resp.Status)
+}
+
 // isPerfFile checks if a file is in perf.data format. It also returns false
 // if it encounters an error during the check.
 func isPerfFile(path string) bool {

internal/symbolizer/symbolizer.go

 	if err != nil {
 		return nil, fmt.Errorf("http post %s: %v", source, err)
 	}
+	defer resp.Body.Close()
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("server response: %s", resp.Status)
+		return nil, statusCodeError(resp)
 	}
-	defer resp.Body.Close()
 	return ioutil.ReadAll(resp.Body)
 }
 
+func statusCodeError(resp *http.Response) error {
+	if resp.Header.Get("X-Go-Pprof") != "" && strings.Contains(resp.Header.Get("Content-Type"), "text/plain") {
+		// error is from pprof endpoint
+		body, err := ioutil.ReadAll(resp.Body)
+		if err == nil {
+			return fmt.Errorf("server response: %s - %s", resp.Status, body)
+		}
+	}
+	return fmt.Errorf("server response: %s", resp.Status)
+}
+
 // doLocalSymbolize adds symbol and line number information to all locations
 // in a profile. mode enables some options to control
 // symbolization.