Notify user when broken symlinks are encountered

[jamesls]

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.

[toastdriven]

I don't understand how the _needs_newline change fits in, but otherwise everything looks fine. 🚢 it.

[jamesls]

When there's an abnormal termination (as in the case here where we have a bad symlink), we'll hit the case where we have a 'message\r' with no newline. This means that means that the prompt will have leftover junk when the process exists. By tracking the newline we can ensure that even in abnormal termination we'll print a proper/readable error message.

[jamesls]

Rebased against develop.

[garnaat]

LGTM