Env.load_box! gives a reasonable error if the box specified in the Vagrantfile doesn't exist now.

lib/vagrant/env.rb

@@ -60,9 +60,18 @@ module Vagrant
       end
 
       def load_box!
-        @@box = Box.find(Vagrant.config.vm.box) if Vagrant.config.vm.box
+        if Vagrant.config.vm.box
+          @@box = Box.find(Vagrant.config.vm.box)
+
+          if !@@box
+            error_and_exit(<<-msg)
+Specified box `#{Vagrant.config.vm.box}` does not exist!
+
+The box must be added through the `vagrant box add` command. Please view
+the documentation associated with the command for more information.
+msg
+          end
 
-        if @@box
           logger.info("Reloading configuration to account for loaded box...")
           load_config!
         end

test/vagrant/env_test.rb

@@ -229,6 +229,15 @@ class EnvTest < Test::Unit::TestCase
       Vagrant::Env.stubs(:load_config!)
     end
 
+    should "not load the box if its not set" do
+      mock_config do |config|
+        config.vm.box = nil
+      end
+
+      Vagrant::Box.expects(:find).never
+      Vagrant::Env.load_box!
+    end
+
     should "set the box to what is found by the Box class" do
       Vagrant::Box.expects(:find).with(Vagrant.config.vm.box).once.returns(@box)
       Vagrant::Env.load_box!
@@ -241,9 +250,9 @@ class EnvTest < Test::Unit::TestCase
       Vagrant::Env.load_box!
     end
 
-    should "not load the config if a box is not loaded" do
-      Vagrant::Env.expects(:load_config!).never
+    should "error and exit if a box is specified but doesn't exist" do
       Vagrant::Box.expects(:find).returns(nil)
+      Vagrant::Env.expects(:error_and_exit).once
       Vagrant::Env.load_box!
     end
   end