src/Pure/Build/build.scala

             no_build = no_build, session_setup = session_setup,
             jobs = max_jobs.getOrElse(if (build_hosts.nonEmpty) 0 else 1), master = true)
 
         if (clean_build) {
           for (name <- full_sessions.imports_descendants(full_sessions_selection)) {
-            store.clean_output(database_server, name) match {
-              case None =>
-              case Some(true) => progress.echo("Cleaned " + name)
-              case Some(false) => progress.echo(name + " FAILED to clean")
-            }
+            store.clean_output(database_server, name, progress = progress)
           }
         }
 
         val results = engine.run_build_process(build_context, progress, server)