Raise a useful exception when an invalid callable is ran

lib/vagrant/action.rb

@@ -45,6 +45,7 @@ module Vagrant
     def run(callable, options=nil)
       callable = Builder.new.use(callable) if callable.kind_of?(Class)
       callable = self.class.actions[callable] if callable.kind_of?(Symbol)
+      raise Exceptions::UncallableAction.new(callable) if !callable
 
       action_environment = Action::Environment.new(env)
       action_environment.merge!(options || {})

lib/vagrant/exceptions/uncallable_action.rb

@@ -0,0 +1,17 @@
+module Vagrant
+  module Exceptions
+    # Raised when an action sequence is trying to be run for an uncallable
+    # action (not a lambda, middleware, or registered sequence).
+    class UncallableAction < Exception
+      def initialize(callable)
+        super()
+
+        @callable = callable
+      end
+
+      def to_s
+        @callable.inspect
+      end
+    end
+  end
+end

test/vagrant/action_test.rb

@@ -31,6 +31,11 @@ class ActionTest < Test::Unit::TestCase
       @klass.actions.clear
     end
 
+    should "raise an exception if a nil action is given" do
+      assert_raises(Vagrant::Exceptions::UncallableAction) { @instance.run(nil) }
+      assert_raises(Vagrant::Exceptions::UncallableAction) { @instance.run(:dontexist) }
+    end
+
     should "run the callable item with the proper context" do
       callable = mock("callable")
       callable.expects(:call).with() do |env|