Extension update should provide information in case of xclass migration

We experienced the following problem when performing the update of Extension Repository Application on extensions.xwiki.org: the logs remained stuck several hours when installing some specific XWiki pages defining xclasses.

The issue is that during the install of those xclass it triggered an XClassMigration, which in turns triggered the migration of the xobjects on all concerned pages. But we did not have any information about this underlying process in the logs of the extension update, neither an idea how long it would take to complete (in case of extensions.xwiki.org it took several hours).

So we do need to have more visibility about the process running in the job logs in such case.