Improve error message in headless mode

Provide advise to users who are using a headless version of java on
accident.

Author: fynngodau

key.ui/src/main/java/de/uka/ilkd/key/gui/MainWindow.java

@@ -393,9 +393,11 @@ public static MainWindow getInstance() {
 
     public static MainWindow getInstance(boolean ensureIsVisible) {
         if (GraphicsEnvironment.isHeadless()) {
+            LOGGER.error("Error: KeY started in graphical mode, but no graphical environment present or supported.");
             LOGGER.error(
-                "Error: KeY started in graphical mode, " + "but no graphical environment present.");
-            LOGGER.error("Please use the --auto option to start KeY in batch mode.");
+                    "If this is unexpected, ensure that you are not using a headless version of Java " +
+                    "(or force windowed mode with java parameter -Djava.awt.headless=false).");
+            LOGGER.error("Otherwise, please use the --auto option to start KeY in batch mode.");
             LOGGER.error("Use the --help option for more command line options.");
             System.exit(-1);
         }