Document How the ProgressBar on Console Uses stderr

[d42ohpaz]

I feel that a note should be added to the ProgressBar documentation stating the console output will always be sent through stderr. This is a subtle nuance for anybody who treat stderr as an error, similar to how a return code > 0 would indicate a failure of a command.

Use case: I use cronic to wrap command execution for cron jobs so that any error output, or result code > 0, is treated as a failure and an email be sent notifying developers of a problem. cronic relies on the existence of either output in stderr or a non-zero result code to make this determination. Suffice it to say, I've been spinning my wheels trying to figure out why my commands are failing successfully.

[chuckhoupt]

Many console programs suppress progress bars when running non-interactively. Possibly ProgressBar could be modified to be quiet when non-interactive? I believe this could be done with posix_isatty, at least on Unix-y systems.