I wasn't aware this is an auto-generated file. I can add a comment but looking at it, it seems we auto-generate this file just to save a call to _dosmaperr. I would refactor the whole function to call _dosmaperr first then if result is still EINVAL, tweak with custom switch case. The way I see it, this looks like premature optimization since OS error shouldn't be on a hot code path, meaning an application should be able to live with an extra CRT function call on such exceptions. I'm willing to implement this if  there are no objections. Something like:

errno = _dosmaperr(err)
if (EINVAL == errno)
{
    switch (err)
    {
    // Our tweaks
    }
}