path/filepath: document that Glob ignores i/o errors

Fixes #8008.

LGTM=adg
R=golang-codereviews, nightlyone, adg
CC=golang-codereviews
https://golang.org/cl/138630045

src/path/filepath/match.go

@@ -228,6 +228,9 @@ func getEsc(chunk string) (r rune, nchunk string, err error) {
 // as in Match. The pattern may describe hierarchical names such as
 // /usr/*/bin/ed (assuming the Separator is '/').
 //
+// Glob ignores file system errors such as I/O errors reading directories.
+// The only possible returned error is ErrBadPattern, when pattern
+// is malformed.
 func Glob(pattern string) (matches []string, err error) {
 	if !hasMeta(pattern) {
 		if _, err = os.Lstat(pattern); err != nil {
@@ -283,10 +286,7 @@ func glob(dir, pattern string, matches []string) (m []string, e error) {
 	}
 	defer d.Close()
 
-	names, err := d.Readdirnames(-1)
-	if err != nil {
-		return
-	}
+	names, _ := d.Readdirnames(-1)
 	sort.Strings(names)
 
 	for _, n := range names {