More verbose error message when ansible-playbook fails

I've spent 1 hour today debugging why packer does not want to work with
ansible. It turns out `ansible-playbook` command was returning non-zero
exit status because of the file system permission problem.

Output before change:

    % packer build rabbitmq.json
    amazon-ebs output will be in this color.

    1 error(s) occurred:

    * exit status 1

Output after change:

    amazon-ebs output will be in this color.

    1 error(s) occurred:

    * Error running "ansible-playbook --version": exit status 1

provisioner/ansible/provisioner.go

@@ -144,7 +144,8 @@ func (p *Provisioner) Prepare(raws ...interface{}) error {
 func (p *Provisioner) getVersion() error {
 	out, err := exec.Command(p.config.Command, "--version").Output()
 	if err != nil {
-		return err
+		return fmt.Errorf(
+			"Error running \"%s --version\": %s", p.config.Command, err.Error())
 	}
 
 	versionRe := regexp.MustCompile(`\w (\d+\.\d+[.\d+]*)`)

provisioner/ansible/provisioner_test.go

@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path"
+	"strings"
 	"testing"
 
 	"github.com/mitchellh/packer/packer"
@@ -258,6 +259,18 @@ func TestAnsibleGetVersion(t *testing.T) {
 	}
 }
 
+func TestAnsibleGetVersionError(t *testing.T) {
+	var p Provisioner
+	p.config.Command = "./test-fixtures/exit1"
+	err := p.getVersion()
+	if err == nil {
+		t.Fatal("Should return error")
+	}
+	if !strings.Contains(err.Error(), "./test-fixtures/exit1 --version") {
+		t.Fatal("Error message should include command name")
+	}
+}
+
 func TestAnsibleLongMessages(t *testing.T) {
 	if os.Getenv("PACKER_ACC") == "" {
 		t.Skip("This test is only run with PACKER_ACC=1 and it requires Ansible to be installed")

provisioner/ansible/test-fixtures/exit1

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+exit 1