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.