cmd: Ensure mkdir returns an error for pre-existing directories

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=11601

Your paranoid android.


=== WVISTAADM (32 bit) ===
batch: unhandled exception c0000005 at 77496FCF

=== W2K8SE (32 bit) ===
batch: unhandled exception c0000005 at 778A8915

=== W7PRO (32 bit) ===
batch.c:70: Test failed: CreateFile failed
batch: unhandled exception c0000005 at 772D3067

=== W7PROX64 (32 bit) ===
batch.c:96: Test failed: CreateProcess failed: 1450

=== W7PROX64 (64 bit) ===
Timeout