Please make coreconverter.exe return code be non=zero in event of error.

Currently the lack of a return code indicating error makes CoreConverter.exe not properly usable on many environment. It is standard for command-line programs to indicate errors in the return code. Since it doesn't to that, CoreConverter.exe doesn't fit properly in everything from MAKE to PowerShell.

Workaround: code a wrapper that

1 creates a unique temporary filename
2 passes it by -errorfile=
3 returns non-zero if thw temp file is non-empty
4 deletes the temp file
5 risks fail if anything goes wrong with the tempfile e.g. out of disc space.