XDCR - BrokenMap events should output a "summary" errorMsg

Per changes of MB-43352, broken map events do not utilize the "ErrorMsg" field of the "ErrorInfo" struct, which is then parsed by ns_server and then output to the UI.

The original goal was that the UI can intelligently parse the EventInfo struct and display brokenMap in a nice fashion (per MB-41218)... and as a result, the "ErrorMsg" field was not populated. This leads to the "empty" error messages shown per https://issues.couchbase.com/browse/MB-41248?focusedCommentId=492157&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-492157

Ideally though, the "ErrorMsg" field should still showcase an error message of some sort. This will ensure that the legacy UI will not just show an empty time without a message.
This also will act as a safety net for CC... if MB-41218 does not make it to 7.0 (per conversation with UI team), a very basic text message showing the broken maps (just like how it shows up in the logs) can still showcase at least some information.