Implemented EZP-23420: Log error if source image doesn't exist

Implements: https://jira.ez.no/browse/EZP-23420

We can consider this as a follow up to https://github.com/ezsystems/ezpublish-kernel/pull/706

In this case, [find](https://github.com/ezsystems/ezpublish-kernel/blob/master/eZ/Bundle/EzPublishCoreBundle/Imagine/BinaryLoader.php#L49) function of `Imagine\BinaryLoader` throws that `NotLoadableException`

This patch catch it and just log the exception message
```php
throw new SourceImageNotFoundException( $originalPath, 0, $e );
```

Ref http://php.net/manual/en/language.exceptions.extending.php .

This allows more readable exceptions, as you'll see that original one was an `NotLoadableException`.