Notify user when broken symlinks are encountered

You will now see an error message shown as well as a non zero
RC:

```
  $ aws s3 sync anotherdir/ s3:/bucket-name/
  [Errno 2] No such file or directory: '/private/tmp/symlnk/anotherdir/z-badsylmink'

  $ echo $?
  1
```

There is potential to add something like a ``--skip-bad-symlinks``
option, but the default behavior is to let the user know that we've hit
a bad symlink.  Fies #425 and #487.