Improving error messages

[mauritskorse]

Right now I sometimes get an error message 'Could not find panel...'.
It would be great if these could be more informative by adding the panel name that was not found.
i.e.

    let panelEl = container.getPanel(panel);
-   if (!panelEl) throw new Error("Could not find panel...");
+   if (!panelEl) throw new Error("Could not find panel "+ panel);