Add echoerr which outputs to STDERR rather than STDOUT for errors

[sklirg]

This fix makes errors go to STDERR rather than STDOUT, which they should.

(By piping the output of this script to /dev/null, you'll only get notified about the errors, as stderr is not sent to /dev/null/)

Resolves #172

[melwil]

All errors are now output to stderr, like they should be. And regular input can be ignored if so desired by piping stdout do /dev/null, as @sklirg said.

We've already tested these changes with our setup and confirmed they work.