Improved JSON parse error log messages

This pull request improves the verbosity of log messages when a JSON parse error happens. If a user (like me) mistypes a basic auth password, elasticdump gives a cryptic error message that doesn't make the problem immediately obvious.

### Before

```
Fri, 05 Dec 2014 20:05:04 GMT | starting dump
Fri, 05 Dec 2014 20:05:04 GMT | got 1 objects from source elasticsearch (offset: 0)
Fri, 05 Dec 2014 20:05:04 GMT | {}
Fri, 05 Dec 2014 20:05:04 GMT | dump ended with error
```
```
Fri, 05 Dec 2014 20:06:59 GMT | starting dump
Fri, 05 Dec 2014 20:06:59 GMT | got 1 objects from source elasticsearch (offset: 0)

SyntaxError: Unexpected token <
    at Object.parse (native)
    at Object.module.exports.parse (/Users/brianreavis/Repositories/elasticsearch-dump/lib/json.js:2:14)
    at elasticsearch.setMapping (/Users/brianreavis/Repositories/elasticsearch-dump/lib/transports/elasticsearch.js:142:18)
    at elasticsearch.set (/Users/brianreavis/Repositories/elasticsearch-dump/lib/transports/elasticsearch.js:131:14)
    at /Users/brianreavis/Repositories/elasticsearch-dump/elasticdump.js:80:17
    at Request._callback (/Users/brianreavis/Repositories/elasticsearch-dump/lib/transports/elasticsearch.js:38:13)
    at Request.self.callback (/Users/brianreavis/Repositories/elasticsearch-dump/node_modules/request/request.js:373:22)
    at Request.EventEmitter.emit (events.js:98:17)
    at Request.<anonymous> (/Users/brianreavis/Repositories/elasticsearch-dump/node_modules/request/request.js:1318:14)
    at Request.EventEmitter.emit (events.js:117:20)
```

### After

```
Fri, 05 Dec 2014 20:22:03 GMT | starting dump
Fri, 05 Dec 2014 20:22:03 GMT | got 1 objects from source elasticsearch (offset: 0)
Fri, 05 Dec 2014 20:22:03 GMT | failed to parse json (message: "Unexpected token <") - source: "<html>\r\n<head><title>401 Authorization Required</title></head>\r\n<body bgcolor=\"white\">\r\n<center><h1>401 Authorization Required</h1></center>\r\n<hr><center>nginx/1.6.2</center>\r\n</body>\r\n</html>\r\n"
Fri, 05 Dec 2014 20:22:03 GMT | dump ended with error
```