Throw an error if a box already exists when calling box.add

BRANCH_TODO

@@ -4,5 +4,4 @@ This is a TODO file for only this branch (config-overhaul)
 * Only allow one kind of vagrantfile to be loaded (i.e. if both
   Vagrantfile and vagrantfile exist, throw an error)
 * Nicer error if can't setup home directory
-* vagrant box add should error if box already exists
 * Default name to folder name + timestamp

lib/vagrant/box_collection.rb

@@ -33,6 +33,8 @@ module Vagrant
     # Adds a box to this collection with the given name and located
     # at the given URL.
     def add(name, url)
+      raise Errors::BoxAlreadyExists, :name => name if find(name)
+
       @action_runner.run(:box_add,
                          :box_name => name,
                          :box_url => url,

test/unit/vagrant/box_collection_test.rb

@@ -31,6 +31,11 @@ describe Vagrant::BoxCollection do
     end
   end
 
+  it "should throw an error if the box already exists when adding" do
+    environment.box("foo")
+    expect { instance.add("foo", "bar") }.to raise_error(Vagrant::Errors::BoxAlreadyExists)
+  end
+
   it "should add the box" do
     name = "foo"
     url  = "bar"