Raise exception if the insert_before middleware is not found

lib/vagrant/action/builder.rb

@@ -57,6 +57,7 @@ module Vagrant
       # given middleware object.
       def insert(index, middleware, *args, &block)
         index = self.index(index) unless index.is_a?(Integer)
+        raise "no such middleware to insert before: #{index.inspect}" unless index
         stack.insert(index, [middleware, args, block])
       end
 

test/unit/vagrant/action/builder_test.rb

@@ -113,7 +113,12 @@ describe Vagrant::Action::Builder do
       data[:data].should == [1, 2, 3]
     end
 
-    it "raises an exception if an invalid object given" do
+    it "raises an exception if an invalid object given for insert" do
+      expect { instance.insert "object", appender_proc(1) }.
+        to raise_error(RuntimeError)
+    end
+
+    it "raises an exception if an invalid object given for insert_after" do
       expect { instance.insert_after "object", appender_proc(1) }.
         to raise_error(RuntimeError)
     end