wkfs/gcs: on Open, return os.ErrNotExist when storage.ErrObjectNotExi…

…st occurs

And document wkfs.Open accordingly.

wkfs/gcs/gcs.go

@@ -102,6 +102,9 @@ func (fs *gcsFS) Open(name string) (wkfs.File, error) {
 	obj := fs.sc.Bucket(bucket).Object(fileName)
 	attrs, err := obj.Attrs(fs.ctx)
 	if err != nil {
+		if err == storage.ErrObjectNotExist {
+			return nil, os.ErrNotExist
+		}
 		return nil, err
 	}
 	size := attrs.Size

wkfs/wkfs.go

@@ -48,6 +48,10 @@ type FileWriter interface {
 	io.Closer
 }
 
+// Open opens the object designated by name, for reading and/or writing as a
+// file. The returned error can be checked with
+// https://golang.org/pkg/os/#IsNotExist to ascertain whether the object actually
+// exists.
 func Open(name string) (File, error)               { return fs(name).Open(name) }
 func Stat(name string) (os.FileInfo, error)        { return fs(name).Stat(name) }
 func Lstat(name string) (os.FileInfo, error)       { return fs(name).Lstat(name) }