Due to the hack done in T184948 Pywikibot crashed with this nice error.
pywikibot.data.api.APIError: failed-save: The save has failed. [help:See https://www.wikidata.org/w/api.php for API usage. Subscribe to the mediawiki-api-announce mailing list at <https://lists.wikimedia.org/mailman/listinfo/mediawiki-api-announce> for notice of API deprecations and breaking changes.; messages:[{'name': 'wikibase-api-failed-save', 'parameters': [], 'html': {'*': 'The save has failed.'}}, {'name': 'actionthrottledtext', 'parameters': [], 'html': {'*': 'As an anti-abuse measure, you are limited from performing this action too many times in a short space of time, and you have exceeded this limit.\nPlease try again in a few minutes.'}}]] <class 'pywikibot.data.api.APIError'> CRITICAL: Closing network session.
Same hack also broke massmessage, see T192690. The hack was removed so the chance of someone running into this is much lower.
Pywikibot should be updated so that a throttle error is a recoverable error. So the usual logic should kick in: Wait a bit, try again.