Improve "unknown command" error

Clarify it is a `brew` command.

Fixes #17627

Library/Homebrew/brew.rb

@@ -116,7 +116,7 @@ begin
       end
       # 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}"
+      odie "Unknown command: brew #{cmd}"
     end
 
     # Unset HOMEBREW_HELP to avoid confusing the tap

Library/Homebrew/cmd/command.rb

@@ -19,7 +19,7 @@ module Homebrew
       def run
         args.named.each do |cmd|
           path = Commands.path(cmd)
-          odie "Unknown command: #{cmd}" unless path
+          odie "Unknown command: brew #{cmd}" unless path
           puts path
         end
       end

Library/Homebrew/cmd/options.rb

@@ -37,7 +37,7 @@ module Homebrew
           puts_options(Formula.installed.sort)
         elsif args.command.present?
           cmd_options = Commands.command_options(T.must(args.command))
-          odie "Unknown command: #{args.command}" if cmd_options.nil?
+          odie "Unknown command: brew #{args.command}" if cmd_options.nil?
 
           if args.compact?
             puts cmd_options.sort.map(&:first) * " "