[GH-514] Improved exception when a type hint class can not be found

This issue is created automatically through a Github pull request on behalf of arnaud-lb:

Url: https://github.com/doctrine/doctrine2/pull/514

Message:

This improves the exception message when a type hint class was not found during the generation of a proxy.

Example:

``` php
<?php
class Foo
{
function test(\InvalidClass $param)
{
}
}
```

When generating a proxy for this class `Foo`, if the type hint class `InvalidClass` cannot be found, a `ReflectionException` exception is thrown with the following message: `Class InvalidClass does not exist`. There is no hint on the real cause of the error (which class, method, ...).

This PR wraps this exception with a message making the cause of the exception more obvious.