Write drudg errors to stderr

[wehimwich]

It would be helpful it drudg wrote errors/warnings to stderr instead of stdout. This would make it possible to see when there is a problem when it is run by fesh or other script that redirects stdout.

Setting the exit code to indicate that a processing error occurred would also be useful so a script can abort further processing if there was an error.

Some care may be needed to separate errors and serious warnings from less serious warnings and informational messages to make sure the stderr output is succinct and only for serious issues, i.e., when the processing is broken.