change wrapped exception message to be more usefull

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

When you try to parse a yml file with the XMLLoader you get the following Exception

```
[Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]
[ERROR 4] Start tag expected, '<' not found (in n/a - line 1, column 1)

[InvalidArgumentException]
[ERROR 4] Start tag expected, '<' not found (in n/a - line 1, column 1)
```

after the patch
```
[Symfony\Component\DependencyInjection\Exception\InvalidArgumentException]      
Unable to parse file "/home/.../src/Application/FOS/UserBundle/DependencyInjection/../Resources/config/services.yml".

[InvalidArgumentException]
[ERROR 4] Start tag expected, '<' not found (in n/a - line 1, column 1)
```