Add helpful error message when attempting to run brew cask

Library/Homebrew/brew.rb

@@ -86,6 +86,10 @@ begin
   require "commands"
   require "settings"
 
+  # Print an error message and exit if the command is no longer supported
+  unsupported_cmd_message = Commands.unsupported_cmd?(cmd)
+  odie unsupported_cmd_message if unsupported_cmd_message
+
   internal_cmd = Commands.valid_internal_cmd?(cmd) || Commands.valid_internal_dev_cmd?(cmd) if cmd
 
   unless internal_cmd

Library/Homebrew/commands.rb

@@ -30,6 +30,13 @@ module Commands
     "lc"          => "livecheck",
     "tc"          => "typecheck",
   }.freeze
+  HOMEBREW_UNSUPPORTED_COMMAND_MESSAGES = {
+    "cask" => "`brew cask <command>` is no longer supported. Use `brew <command> --cask` instead.",
+  }.freeze
+
+  def unsupported_cmd?(cmd)
+    HOMEBREW_UNSUPPORTED_COMMAND_MESSAGES.fetch(cmd, nil)
+  end
 
   def valid_internal_cmd?(cmd)
     require?(HOMEBREW_CMD_PATH/cmd)