Instead of generating our own "Maybe you meant ..." message, when a
generator or a server isn't found, use DidYouMean::Correctable.

This commit adds a generic CorrectableError in the Command namespace
that could be used for corrections for other commands.

By using DidYouMean we can delete the Command::Spellchecker.

Co-authored-by: Ryuta Kamizono <kamipo@gmail.com>