Add additional info in cases where Wiktionary does not have a page with useful results

[bjkeefe]

Involves several small changes to core.py, so that, when applicable, .fetch() will return information distinguishing between two cases: (1) there is no Wiktionary entry for for the default language and (2) there is no Wiktionary entry for at all.

Modifications are in a new branch named additional_info.

Details in readme_modifications.md, in the same directory as readme.md:

https://github.com/bjkeefe/WiktionaryParser/blob/additional_info/readme_modifications.md

Please note: this is my first pull request, so I apologize in advance if I haven't done some things correctly. Please feel to offer any advice. And of course, please feel free to follow up regarding the source code modifications themselves. I'm happy to make adjustments. My email address is in readme_modifications.md, if you don't see it anywhere else.

Thanks for considering!