Does not handle loading failures

[gollux]

When I try to load a document, which is not a PDF, load_from_file succeeds, but all subsequent methods crash. The reason is that self._document becomes None and most subsequent methods are decorated with @ensure_unlocked, leading to a call of self._document.is_locked.

I am willing to fix it, but which solution do you prefer? We can add an is_broken method, which can be used after loading the document to check if it was loaded successfully. Or we can make loading throw an exception upon failure.