cmd/pprof: restore printing descriptive errors from net/http/pprof endpoints

Restores functionality added in https://golang.org/cl/35564/ which was
lost in https://golang.org/cl/36798/ by the addition of the custom
fetcher to src/cmd/pprof/pprof.go. The custom fetcher overrides the
upstream default.

Change-Id: Ic71e5e475d043276d916298ab5acb5c9b9ad063e
Reviewed-on: https://go-review.googlesource.com/45812Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/pprof/pprof.go

@@ -13,11 +13,13 @@ import (
 	"crypto/tls"
 	"debug/dwarf"
 	"fmt"
+	"io/ioutil"
 	"net/http"
 	"net/url"
 	"os"
 	"regexp"
 	"strconv"
+	"strings"
 	"sync"
 	"time"
 
@@ -82,11 +84,22 @@ func getProfile(source string, timeout time.Duration) (*profile.Profile, error)
 		return nil, err
 	}
 	if resp.StatusCode != http.StatusOK {
-		return nil, fmt.Errorf("server response: %s", resp.Status)
+		defer resp.Body.Close()
+		return nil, statusCodeError(resp)
 	}
 	return profile.Parse(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
+		if body, err := ioutil.ReadAll(resp.Body); err == nil {
+			return fmt.Errorf("server response: %s - %s", resp.Status, body)
+		}
+	}
+	return fmt.Errorf("server response: %s", resp.Status)
+}
+
 // cpuProfileHandler is the Go pprof CPU profile handler URL.
 const cpuProfileHandler = "/debug/pprof/profile"