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.