Fix loading deployer of Heroku and OpenShift.

I wrote a blog about how I [Fixed Hexo's Deployer.](http://nicktd.com/2014/04/25/fixing-hexos-deployer/)

#### Before this fix:
When trying to deploy blog to Heroku error happens, because the deployer for Heroku isn't loaded.
Same for OpenShift's deployer.

#### Suggestion:
In `hexo/lib/loaders/entend.js`, a logging could be added for errors at 31th line in the catch statement. Because if not the error is hidden by that catch statement.
Here's what I added when I was debugging:

``` javascript
try {
   require('../plugins/' + item);
} catch (e){
  hexo.log.e("Failed loading: " + item + " for: " + e);
}
```
<br>
If you need anything, just leave me a comment. :smile:

#### PS.
Hexo is AWESOME!  Great work!
Thanks.