Fix error dialog being displayed in a headless context.

This was causing an infinite cycle of java.awt.HeadlessException in case handleError was called in an environment without display.

jme3-core/src/main/java/com/jme3/app/Application.java

@@ -518,13 +518,15 @@ public class Application implements SystemListener {
     public void handleError(String errMsg, Throwable t){
         // Print error to log.
         logger.log(Level.SEVERE, errMsg, t);
-        // Display error message on screen
+        // Display error message on screen if not in headless mode
+        if (context.getType() != JmeContext.Type.Headless) {
             if (t != null) {
                 JmeSystem.showErrorDialog(errMsg + "\n" + t.getClass().getSimpleName() +
                         (t.getMessage() != null ? ": " +  t.getMessage() : ""));
             } else {
                 JmeSystem.showErrorDialog(errMsg);
             }
+        }
 
         stop(); // stop the application
     }