Throw an exception when mockery_init is not found

I seem to be coming across this error quite often, it happens when the class has already been loaded. There are a lot of discussion around this error online. The error i get is:

```
Call to undefined method RandomClass::mockery_init() in /vendor/composer/mockery/mockery/library/Mockery/Container.php on line 219
```

The reason for this PR is to try and cleanup the output when this error happens. The exception message may not be correct, so feel free to suggest another, but i feel that having an exception here instead of just the PHP error will clear things up. 

Thanks
Scott