Added leading slashes to @expectedException definitions

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: -
Todo: -
License of the code: MIT
Documentation PR: -

This PR adds leading backslashes `\` to `@expectedException` declarations.

The current version is inconsistent with other class references in doc blocks, which default to *relative* class names unless class name has a leading backslash.

```php
/**
 * @param RelativeNs\Class $param1
 * @param \AbsoluteNs\Class $param2
 */

but

/**
 * @expectedException AbsoluteNs\Class
 */
```

Consequently, PHPStorm does not understand the current statements and marks them as erroneous (and correctly so IMO, even though PHPUnit's interpretation is more relaxed).