Errors encountered by app client libraries, should be submitted to the event platform as client error events so that we can be alerted to problems without affecting the end user experience. Such errors might include stream misconfigurations as well as things like HTTP errors.

Both app client libraries will need updates to support configurable destination event services. This already mostly exists in the Java library but will need to be added from scratch in the Swift library.

Do we need a dedicated stream and/or schema for this? mediawiki.client.error (stream) and mediawiki/client/error (schema) don't seem quite on point for the apps, but could be used.

The MediaWiki-oriented libraries (PHP & JS) will make use of native MediaWiki error logging.