Should return an error when the box file is missing

post-processor/vagrant-cloud/post-processor.go

@@ -226,5 +226,11 @@ func providerFromBuilderName(name string) string {
 // Returns the Vagrant provider the box is intended for use with by
 // reading the metadata file packaged inside the box
 func providerFromVagrantBox(boxfile string) (providerName string, err error) {
+	f, err := os.Open(boxfile)
+	if err != nil {
+		return "", fmt.Errorf("Error attempting to open box file: %s", err)
+	}
+	defer f.Close()
+
 	return "", nil
 }

post-processor/vagrant-cloud/post-processor_test.go

@@ -197,3 +197,12 @@ func TestProviderFromBuilderName(t *testing.T) {
 		t.Fatal("should convert provider")
 	}
 }
+
+func TestProviderFromVagrantBox_missing_box(t *testing.T) {
+	boxfile := "i_dont_exist.box"
+	_, err := providerFromVagrantBox(boxfile)
+	if err == nil {
+		t.Fatal("Should have error as box file does not exist")
+	}
+	t.Logf("%s", err)
+}