Unknown Role directive should state the file and line of the unknown role

```
PHP Fatal error:  Uncaught exception 'Exception' with message 'Unknown directive: role' in /Users/kay/Development/rest/vendor/gregwar/rst/Gregwar/RST/ErrorManager.php:17
Stack trace:
```

I would expect something like:

```
unknown directive role in file whatever.rst on line 510
```
