Proxy Autoloader: autoloader assumes $notFoundCallback always creates the proxy file

I saw this while trying to figure out https://github.com/doctrine/DoctrineBundle/issues/848. In this case, the autoloader was called to load a proxy for a mapped superclass. Since the file didn't exist, the `$notFoundCallback` was called to generate a proxy class. However, the `ProxyFactory` in ORM will skip over mapped superclasses and didn't create a class, but neither threw an exception.

This causes an error because the autoloader wrongly assumes that the file will be present, at least after invoking the `$notFoundCallback`: https://github.com/doctrine/common/blob/master/lib/Doctrine/Common/Proxy/Autoloader.php#L74. However, this assumption is wrong and should probably be checked, with an exception being thrown if no file was generated/found.

Start with lib/Doctrine/Common/Proxy/Autoloader.php at line 74 and trace how the autoloader handles a missing proxy file and invokes $notFoundCallback. Compare that flow with the ORM ProxyFactory behavior for mapped superclasses; the work is done when a callback that creates no file is detected and produces the intended exception rather than continuing with a missing file.

Written by the indexing model from the issue text.