check if a box exists before beginning add action

lib/vagrant/actions/box/add.rb

@@ -3,6 +3,10 @@ module Vagrant
     module Box
       class Add < Base
         def prepare
+          if File.exists?(@runner.directory)
+            raise ActionException.new("A box with the name '#{@runner.name}' already exists, please use another name or use `vagrant box remove #{@runner.name}`")
+          end
+          
           @runner.add_action(Download)
           @runner.add_action(Unpackage)
         end

test/vagrant/actions/box/add_test.rb

@@ -3,6 +3,8 @@ require File.join(File.dirname(__FILE__), '..', '..', '..', 'test_helper')
 class AddBoxActionTest < Test::Unit::TestCase
   setup do
     @runner, @vm, @action = mock_action(Vagrant::Actions::Box::Add)
+    @runner.stubs(:directory).returns("foo")
+    File.stubs(:exists?).returns(false)
     mock_config
   end
 
@@ -23,4 +25,15 @@ class AddBoxActionTest < Test::Unit::TestCase
       @action.prepare
     end
   end
+
+  context "providing a name for a base that exists" do
+    should "result in an action exception" do
+      File.expects(:exists?).once.returns(true)
+      @runner.expects(:name).twice.returns('foo')
+      @runner.expects(:add_action).never
+      assert_raise Vagrant::Actions::ActionException do
+        @action.prepare
+      end
+    end
+  end
 end