net/mail: propagate unsupported charset error

Fixes #6807.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/95060043

src/pkg/net/mail/message.go

@@ -363,7 +363,7 @@ func (p *addrParser) consumePhrase() (phrase string, err error) {
 	// Ignore any error if we got at least one word.
 	if err != nil && len(words) == 0 {
 		debug.Printf("consumePhrase: hit err: %v", err)
-		return "", errors.New("mail: missing word in phrase")
+		return "", fmt.Errorf("mail: missing word in phrase: %v", err)
 	}
 	phrase = strings.Join(words, " ")
 	return phrase, nil
@@ -442,11 +442,11 @@ func (p *addrParser) len() int {
 func decodeRFC2047Word(s string) (string, error) {
 	fields := strings.Split(s, "?")
 	if len(fields) != 5 || fields[0] != "=" || fields[4] != "=" {
-		return "", errors.New("mail: address not RFC 2047 encoded")
+		return "", errors.New("address not RFC 2047 encoded")
 	}
 	charset, enc := strings.ToLower(fields[1]), strings.ToLower(fields[2])
 	if charset != "iso-8859-1" && charset != "utf-8" {
-		return "", fmt.Errorf("mail: charset not supported: %q", charset)
+		return "", fmt.Errorf("charset not supported: %q", charset)
 	}
 
 	in := bytes.NewBufferString(fields[3])
@@ -457,7 +457,7 @@ func decodeRFC2047Word(s string) (string, error) {
 	case "q":
 		r = qDecoder{r: in}
 	default:
-		return "", fmt.Errorf("mail: RFC 2047 encoding not supported: %q", enc)
+		return "", fmt.Errorf("RFC 2047 encoding not supported: %q", enc)
 	}
 
 	dec, err := ioutil.ReadAll(r)

src/pkg/net/mail/message_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"io/ioutil"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 )
@@ -116,6 +117,14 @@ func TestDateParsing(t *testing.T) {
 	}
 }
 
+func TestAddressParsingError(t *testing.T) {
+	const txt = "=?iso-8859-2?Q?Bogl=E1rka_Tak=E1cs?= <unknown@gmail.com>"
+	_, err := ParseAddress(txt)
+	if err == nil || !strings.Contains(err.Error(), "charset not supported") {
+		t.Errorf(`mail.ParseAddress(%q) err: %q, want ".*charset not supported.*"`, txt, err)
+	}
+}
+
 func TestAddressParsing(t *testing.T) {
 	tests := []struct {
 		addrsStr string