tap: use dedicated exception for already tapped.

Library/Homebrew/cmd/tap.rb

@@ -23,7 +23,7 @@ module Homebrew extend self
 
     # we downcase to avoid case-insensitive filesystem issues
     tapd = HOMEBREW_LIBRARY/"Taps/#{user.downcase}-#{repo.downcase}"
-    raise "Already tapped!" if tapd.directory?
+    raise AlreadyTappedError if tapd.directory?
     abort unless system "git clone https://github.com/#{repouser}/homebrew-#{repo} #{tapd}"
 
     files = []

Library/Homebrew/exceptions.rb

@@ -221,6 +221,11 @@ class CompilerSelectionError < StandardError
   end
 end
 
+# raised in install_tap
+class AlreadyTappedError < RuntimeError
+  def initialize; super "Already tapped!" end
+end
+
 # raised in CurlDownloadStrategy.fetch
 class CurlDownloadStrategyError < RuntimeError; end