QbsProjectManager: Fix infinite loop on per-product build.

- Setting the QFuture result is not enough, we also need to emit the
  finished() signal.
- Never fail silently; use soft asserts for things that shouldn't happen
  and compile pane output for things that can.

Task-number: QBS-705
Change-Id: I4dad18c2ad573a92f2a0c5aa92b596851c56541e
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/qbsprojectmanager/qbscleanstep.cpp

@@ -105,6 +105,7 @@ void QbsCleanStep::run(QFutureInterface<bool> &fi)
 
     if (!m_job) {
         m_fi->reportResult(false);
+        emit finished();
         return;
     }