Add a V2 config helper to merge errors since that seems common

lib/vagrant/config/v2/root.rb

@@ -1,3 +1,5 @@
+require "vagrant/config/v2/util"
+
 module Vagrant
   module Config
     module V2
@@ -54,14 +56,14 @@ module Vagrant
               # we merge them into our total errors list.
               result = instance.validate(machine)
               if result && !result.empty?
-                result.each do |key, value|
-                  if !value.empty?
-                    errors[key] ||= []
-                    errors[key] += value
-                  end
+                errors = Util.merge_errors(errors, result)
               end
             end
           end
+
+          # Go through and delete empty keys
+          errors.keys.each do |key|
+            errors.delete(key) if errors[key].empty?
           end
 
           errors

lib/vagrant/config/v2/util.rb

@@ -0,0 +1,21 @@
+module Vagrant
+  module Config
+    module V2
+      class Util
+        # This merges two error hashes from validate methods.
+        #
+        # @param [Hash] first
+        # @param [Hash] second
+        # @return [Hash] Merged result
+        def self.merge_errors(first, second)
+          first.dup.tap do |result|
+            second.each do |key, value|
+              result[key] ||= []
+              result[key] += value
+            end
+          end
+        end
+      end
+    end
+  end
+end

test/unit/vagrant/config/v2/util_test.rb

@@ -0,0 +1,21 @@
+require File.expand_path("../../../../base", __FILE__)
+
+require "vagrant/config/v2/util"
+
+describe Vagrant::Config::V2::Util do
+  describe "merging errors" do
+    it "should merge matching keys and leave the rest alone" do
+      first  = { "one" => ["foo"], "two" => ["two"] }
+      second = { "one" => ["bar"], "three" => ["three"] }
+
+      expected = {
+        "one" => ["foo", "bar"],
+        "two" => ["two"],
+        "three" => ["three"]
+      }
+
+      result = described_class.merge_errors(first, second)
+      result.should == expected
+    end
+  end
+end