Add stdout and stderr to the error message

The response is currently:

```js
{ [Error: Command failed: ]
  killed: false,
  code: 1,
  signal: null,
  name: 'Error',
  message: 'Command failed: ',
  stack: 'Error: Command failed: \n    at ChildProcess.exithandler (child_process.js:647:15)\n    at ChildProcess.emit (events.js:98:17)\n    at maybeClose (child_process.js:755:16)\n    at Socket.<anonymous> (child_process.js:968:11)\n    at Socket.emit (events.js:95:17)\n    at Pipe.close (net.js:465:12)',
  showStack: false,
  showProperties: true,
  plugin: 'gulp-shell' }
```

It would be nice to have the `stdout` and the `stderr` to be able to display more information (or parse more information.) to get a better understanding about why this executable failed.
The signature matches : http://nodejs.org/api/child_process.html#child_process_child_process_exec_command_options_callback