File box downloader gives a nice error message if the file doesn't exist.

lib/vagrant/actions/box/download.rb

@@ -26,6 +26,9 @@ module Vagrant
           end
 
           raise ActionException.new("Unknown URI type for box download.") unless @downloader
+
+          # Prepare the downloader
+          @downloader.prepare(@runner.uri)
         end
 
         def execute!

lib/vagrant/downloaders/base.rb

@@ -5,6 +5,9 @@ module Vagrant
     class Base
       include Vagrant::Util
 
+      # Called prior to execution so any error checks can be done
+      def prepare(source_url); end
+
       # Downloads the source file to the destination file. It is up to
       # implementors of this class to handle the logic.
       def download!(source_url, destination_file); end

lib/vagrant/downloaders/file.rb

@@ -3,6 +3,16 @@ module Vagrant
     # "Downloads" a file to a temporary file. Basically, this downloader
     # simply does a file copy.
     class File < Base
+      def prepare(source_url)
+        if !::File.file?(source_url)
+          raise Actions::ActionException.new(<<-msg)
+The given box does not exist on the file system:
+
+#{source_url}
+msg
+        end
+      end
+
       def download!(source_url, destination_file)
         FileUtils.cp(source_url, destination_file.path)
       end

test/vagrant/actions/box/download_test.rb

@@ -16,6 +16,10 @@ class DownloadBoxActionTest < Test::Unit::TestCase
       @uri = mock("uri")
       @uri.stubs(:is_a?).returns(false)
       URI.stubs(:parse).returns(@uri)
+
+      @downloader = mock("downloader")
+      Vagrant::Downloaders::File.any_instance.stubs(:prepare)
+      Vagrant::Downloaders::HTTP.any_instance.stubs(:prepare)
     end
 
     should "raise an exception if no URI type is matched" do
@@ -25,6 +29,13 @@ class DownloadBoxActionTest < Test::Unit::TestCase
       }
     end
 
+    should "call #prepare on the downloader" do
+      @downloader.expects(:prepare).with(@runner.uri).once
+      Vagrant::Downloaders::File.expects(:new).returns(@downloader)
+      @uri.stubs(:is_a?).with(URI::Generic).returns(true)
+      @action.prepare
+    end
+
     should "set the downloader to file if URI is generic" do
       @uri.stubs(:is_a?).with(URI::Generic).returns(true)
       @action.prepare

test/vagrant/downloaders/base_test.rb

@@ -11,6 +11,13 @@ class BaseDownloaderTest < Test::Unit::TestCase
     end
 
     should "implement prepare which does nothing" do
+      assert_nothing_raised do
+        assert @base.respond_to?(:prepare)
+        @base.prepare("source")
+      end
+    end
+
+    should "implement download! which does nothing" do
       assert_nothing_raised do
         assert @base.respond_to?(:download!)
         @base.download!("source", "destination")

test/vagrant/downloaders/file_test.rb

@@ -6,6 +6,15 @@ class FileDownloaderTest < Test::Unit::TestCase
     @uri = "foo.box"
   end
 
+  context "preparing" do
+    should "raise an exception if the file does not exist" do
+      File.expects(:file?).with(@uri).returns(false)
+      assert_raises(Vagrant::Actions::ActionException) {
+        @downloader.prepare(@uri)
+      }
+    end
+  end
+
   context "downloading" do
     should "cp the file" do
       path = '/path'