A more helpful error message for brew untap

`brew untap` without arguments currently errors out (as it should), but the
error message is generic and unhelpful. (The error message is borrowed from
a method in the tap command - `tap_args`.)

Closes Homebrew/homebrew#18186.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cmd/untap.rb

@@ -2,6 +2,8 @@ require 'cmd/tap' # for tap_args
 
 module Homebrew extend self
   def untap
+    raise "Usage is `brew untap <tap-name>`" if ARGV.empty?
+
     user, repo = tap_args
 
     # we consistently downcase in tap to ensure we are not bitten by case-insensive