give the user a error msg on open failure.

Change-Id: If4a57bac7f3e1746c2a05c7a96747a38da188041
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/368


Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>