go/scanner: partially revert CL4538096 now that we have %#U for runes

R=r, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/4572049

src/pkg/go/scanner/scanner.go

@@ -22,6 +22,7 @@ package scanner
 
 import (
 	"bytes"
+	"fmt"
 	"go/token"
 	"path/filepath"
 	"strconv"
@@ -674,7 +675,7 @@ scanAgain:
 			tok = S.switch3(token.OR, token.OR_ASSIGN, '|', token.LOR)
 		default:
 			if S.mode&AllowIllegalChars == 0 {
-				S.error(offs, "illegal character "+strconv.QuoteRune(ch))
+				S.error(offs, fmt.Sprintf("illegal character %#U", ch))
 			}
 			insertSemi = S.insertSemi // preserve insertSemi info
 		}

src/pkg/go/scanner/scanner_test.go

@@ -650,9 +650,9 @@ var errors = []struct {
 	pos int
 	err string
 }{
-	{"\a", token.ILLEGAL, 0, "illegal character '\\a'"},
-	{`#`, token.ILLEGAL, 0, "illegal character '#'"},
-	{`…`, token.ILLEGAL, 0, "illegal character '…'"},
+	{"\a", token.ILLEGAL, 0, "illegal character U+0007"},
+	{`#`, token.ILLEGAL, 0, "illegal character U+0023 '#'"},
+	{`…`, token.ILLEGAL, 0, "illegal character U+2026 '…'"},
 	{`' '`, token.CHAR, 0, ""},
 	{`''`, token.CHAR, 0, "illegal character literal"},
 	{`'\8'`, token.CHAR, 2, "unknown escape sequence"},