Properly close the Linux directory watcher when listing a subdirectory fails.

R=rnystrom@google.com
BUG=15279

Review URL: https://codereview.chromium.org//89503002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/watcher@30690 260f80e4-7a28-3924-810f-c04153c831b5