Merge pull request #12708 from phoenixeliot/add-brew-cask-unsupported-message

Add helpful error message when attempting to run `brew cask`

Library/Homebrew/brew.rb

@@ -121,6 +121,8 @@ begin
     possible_tap = Tap.fetch(possible_tap.first) if possible_tap
 
     if !possible_tap || possible_tap.installed? || Tap.untapped_official_taps.include?(possible_tap.name)
+      # Check for cask explicitly because it's very common in old guides
+      odie "`brew cask` is no longer a `brew` command. Use `brew <command> --cask` instead." if cmd == "cask"
       odie "Unknown command: #{cmd}"
     end