More comprehensive error output

The error output from this plugin was not really formatted very well: 
```JavaScript
{ plugin: 'gulp-sass',
  showStack: undefined,
  name: 'Error',
  message: 'sources/scss/forms:20: error: error reading values after display\n',
  fileName: undefined,
  lineNumber: undefined }
```
The function that I added transforms it into
```Javascript
{ plugin: 'gulp-sass',
  showStack: false,
  name: 'Error',
  message: 'error reading values after display',
  fileName: 'forms'}
```

I understand this is not really an issue that is caused by this plugin. But fixing this problem would involve adding JSON error output from node-sass.

I do not think what I made is finished or stable. But at least we can discuss the issue here.