add warning if corpus services are missing

src/main/java/de/uni_hamburg/corpora/gui/GUI.java

@@ -160,6 +160,14 @@ public class GUI {
         });
         mainPanel.add(buttonPanel);
         window.getContentPane().add(mainPanel);
+        if (!new File("corpus-services.jar").exists()) {
+            checkButton.setEnabled(false);
+            JOptionPane.showMessageDialog(window,
+                    "Corpus services file corpus-services.jar is missing. Before you can use this application"
+                    + " you have to update Corpus services using the button below.",
+                    "Warning",
+                    JOptionPane.WARNING_MESSAGE);
+        }
     }
 
     /**