Noticing, tracking and debugging errors should be easy in MediaWiki. A developer or site maintainer should be able to easily notice new errors (possibly only a subset of them that they are interested in), find out details, and share with others.

For unaffiliated wikis which we don't want to invite to use Wikimedia infrastructure this should be already possible via Extension:Sentry and https://getsentry.com/ or a self-hosted Sentry instance, but that needs to be tested, documented, popularized and feedback collected.