text/template: Put bad function name in quotes in panic from (*Template).Funcs

This turns

	panic: function name  is not a valid identifier

into
	panic: function name "" is not a valid identifier

and also makes it consistent with the func signature check.

This CL also makes the testBadFuncName func a test helper.

Change-Id: Id967cb61ac28228de81e1cd76a39f5195a5ebd11
Reviewed-on: https://go-review.googlesource.com/130998
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/text/template/exec_test.go

@@ -1279,6 +1279,7 @@ func TestBadFuncNames(t *testing.T) {
 }
 
 func testBadFuncName(name string, t *testing.T) {
+	t.Helper()
 	defer func() {
 		recover()
 	}()

src/text/template/funcs.go

@@ -65,7 +65,7 @@ func createValueFuncs(funcMap FuncMap) map[string]reflect.Value {
 func addValueFuncs(out map[string]reflect.Value, in FuncMap) {
 	for name, fn := range in {
 		if !goodName(name) {
-			panic(fmt.Errorf("function name %s is not a valid identifier", name))
+			panic(fmt.Errorf("function name %q is not a valid identifier", name))
 		}
 		v := reflect.ValueOf(fn)
 		if v.Kind() != reflect.Func {