Surface Plugin Errors to UI

[DavieReid]

With plugin errors no longer closing sources it would be good to find a way to surface plugin errors to the site somehow.

Perhaps a mosaic dev tools à la https://tanstack.com/query/v4/docs/react/devtools
Perhaps a new middleware
Perhaps we write a plugins-report.json file into the Mosaic global filesystem.

Right now, the plugin errors are grouped by source. But this isn't something a regular user would know about. It would be more intuitive to show all plugin errors for a namespace (which may contain multiple sources). The SourceManager should be able to handle this.