Clean up error reporting in install-winrm.cmd

Based on comments in
https://github.com/misheska/basebox-packer/pull/39#issuecomment-32768677
but I'm seeing non-fatal errors in the winrm installation so I don't think
we want to always fail if we see a non-zero errorlevel.
Also standardized the script for all Windows flavors.
Added `<NUL` to all powershell commands (as I read somewhere that's advisable)