Sass compilation error shows incorrect file when working with imported files.

Often times, SASS code is spread scross a number of files. I have a styles.scss which contains

```
@import 'path/to/my-mixin'
@import 'path/to/my-other-mixin'
```

When compilation of _my-mixin.scss fails, better_errors shows an error message along the lines of:

> Sass error in styles.scss on line 41

This is a problem because it is impossible to know which imported mixin contains the error and if the line number is even correct.