Fix flags in FormatMessage, Wvarargs warning fix

MinGW has been giving a warning about 'err is not last named argument' for the line ```va_start ( arg_ptr, err );```; C standard says that va_start should use the last named argument with ```va_start```. When going to fix this, I noticed the wrong flags have been supplied to FormatMessage, the ```FORMAT_MESSAGE_ARGUMENT_ARRAY``` flag is not supposed to be used with a ```va_list``` structure like it was in the code. After looking at the other places in OCE that use ```_osd_wnt_set_error``` and reading that using ```FORMAT_MESSAGE_FROM_SYSTEM``` shouldn't really be used with the optional argument list unless you're sure of the exact error and what that error message requires in terms of optional inserts, I figured I'd just neuter this function and leave out the optional arguments entirely.