Improve error messages on less render errors

They now include the actual `.less` file which caused the error, along with the correct line number and other helpful things:

```
$ webpack
…
Module build failed: .btn is undefined
 @ …/style.less (line 31, column 6)
 near lines:
       .table-message {
         .btn;
         text-transform: none;
 @ ./akindi/style/style.less 4:2-282
```

Please forgive the lack of tests. I wasn't able to figure out the best way to make the error case play well with the existing suite.
* When an `Error` is returned a full stack trace is printed when the error is displayed, which doesn't seem appropriate. Should I return an `Error` with a nulled-out `.stack`? Or is there some other way to avoid that stack trace?

* I chose not to include the `filename` because, if `e.filename` isn't set, there is no way to include it without potentially being misleading. The original entry point's filename will be included in the output, so duplicating it here would not be useful:

```
Module build failed: Unrecognised input
 @ /…/component.less (line 98, column 0)
 near lines:
   asdf
   
 @ ./site/style.less 4:2-282
```