If no formula but multiple kegs, advise resolution

In this case the user will simply have to delete all but one keg in order to use the command that uses ARGV.kegs.

Refs Homebrew/homebrew#11050.

Library/Homebrew/cmd/unlink.rb

@@ -1,6 +1,6 @@
 module Homebrew extend self
   def unlink
-    raise FormulaUnspecifiedError if ARGV.named.empty?
+    raise KegUnspecifiedError if ARGV.named.empty?
 
     ARGV.kegs.each do |keg|
       print "Unlinking #{keg}... "

Library/Homebrew/extend/ARGV.rb

@@ -16,6 +16,7 @@ module HomebrewArgvExtension
   end
 
   def kegs
+    rack = nil
     require 'keg'
     require 'formula'
     @kegs ||= downcased_unique_named.collect do |name|
@@ -47,6 +48,15 @@ module HomebrewArgvExtension
         Keg.new(linked_keg_ref.realpath)
       end
     end
+  rescue FormulaUnavailableError
+    if rack
+      raise <<-EOS.undent
+        Multiple kegs installed to #{rack}
+        Please remove all but one and then try again.
+      EOS
+    else
+      raise
+    end
   end
 
   # self documenting perhaps?