Make brew tap error message more clear

Closes Homebrew/homebrew#25813.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/tap.rb

@@ -110,7 +110,7 @@ module Homebrew extend self
 
   def tap_args
     ARGV.first =~ %r{^(\S+)/(homebrew-)?(\w+)$}
-    raise "Invalid usage" unless $1 and $3
+    raise "Invalid tap name" unless $1 && $3
     [$1, $3]
   end