Let Editor::statusError print to stderr

Before, these were only shown in the GUI, which makes a failing
commandline build a bit puzzling. As a side effect, the error is now
shown in the log area in addition to the status line above the log
area, but that should be ok.

app/src/processing/app/Editor.java

@@ -2638,6 +2638,7 @@ public class Editor extends JFrame implements RunnerListener {
    * Show an error int the status bar.
    */
   public void statusError(String what) {
+    System.err.println(what);
     status.error(what);
     //new Exception("deactivating RUN").printStackTrace();
     toolbar.deactivate(EditorToolbar.RUN);