Send only the actual error to Console.Error

Currently all generated help text is sent to `stderr` (`Console.Error`) when an error occurs:
https://github.com/commandlineparser/commandline/blob/7aa501d534a657c388f94f66127b4f8dde64ac59/src/CommandLine/Parser.cs#L199

Wouldn't it make more sense to only send the actual error there, and send the rest of the help text (both before and after) to `stdout` (`Console.Out`)?

Read src/CommandLine/Parser.cs around line 199 and trace how help text and errors are written to the console. The change is complete when only the actual error goes to Console.Error, while the help text before and after it goes to Console.Out.

索引モデルが issue の本文から書いたものです。