message: bubble encoding errors to WalkFunc

Only charset errors were forwarded.

entity.go

@@ -164,7 +164,7 @@ func (e *Entity) Walk(walkFunc WalkFunc) error {
  multipartReaders = multipartReaders[:len(multipartReaders)-1]
  path = path[:len(path)-1]
  continue
- } else if IsUnknownCharset(err) {
+ } else if IsUnknownEncoding(err) || IsUnknownCharset(err) {
  // Forward the error to walkFunc
  } else if err != nil {
  return err