Improve uncloneable exception

hey, 

I just improve uncloneable exception, when you don't want to skip uncloneable and you got exception from php clone you got this message:

```
Error: Trying to clone an uncloneable object of class ReflectionClass in /home/.../vendor/myclabs/deep-copy/src/DeepCopy/DeepCopy.php line 132
```

Now:

```
Class "Acme\AcmeDemoBundle\Entity\Acme" is not cloneable.
```

Now it is easy to see which class raises concerns directly :)