Results not updated if a category given in criteria doesn't exist

[jhsoby]

Describe the bug
The bot doesn't update the results if one of the categories given in the criteria doesn't exist.

To Reproduce
Steps to reproduce the behavior:

1. In https://no.wikipedia.org/wiki/Bruker%3ADanmichaelo%2FSandkasse5
2. Add {{Ukens konkurranse kriterium |kategori | Non-existent category}} to the criteria
3. Run the bot on that page: ukbot --page "Bruker:Danmichaelo/Sandkasse5" config/config.no-mk.yml --simulate --output Sandkasse5.txt

What happens:
The bot doesn't update the results, because the category page doesn't exist.

Expected behavior
Categories can still have pages in them even if the category page doesn't exist. It's okay for the bot to add a warning about the category not existing, but that shouldn't stop the whole process.

Additionally, the bot should probably distinguish between categories not existing on a recognized target wiki, and a target wiki not being recognized by the bot (to test this, add zu:Something to the category criteria, assuming zu.wikipedia.org is not added to other_sites in your contest config.

Screenshots
N/A

Desktop (please complete the following information):

• OS: N/A
• Browser: N/A
• Version: We should probably include versioning in the bot output, for easier debugging?