Command line error when GUI required

[stefanogermano]

Describe the bug

When you run WIDOCO through the command line, but an operation requires the GUI (and it cannot be executed), it throws an exception:

[main] ERROR widoco.gui.GuiController - Error while generating the documentation: 
No X11 DISPLAY variable was set,
but this program performed an operation which requires it.
java.awt.HeadlessException: 
No X11 DISPLAY variable was set,
but this program performed an operation which requires it.
        at java.desktop/java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:166)
        at java.desktop/java.awt.Window.<init>(Window.java:553)
        at java.desktop/java.awt.Frame.<init>(Frame.java:428)
        at java.desktop/java.awt.Frame.<init>(Frame.java:393)
        at java.desktop/javax.swing.SwingUtilities$SharedOwnerFrame.<init>(SwingUtilities.java:1919)
        at java.desktop/javax.swing.SwingUtilities.getSharedOwnerFrame(SwingUtilities.java:1995)
        at java.desktop/javax.swing.JOptionPane.getRootFrame(JOptionPane.java:1695)
        at java.desktop/javax.swing.JOptionPane.showOptionDialog(JOptionPane.java:874)
        at widoco.CreateResources.saveDocument(CreateResources.java:394)
        at widoco.CreateResources.createAbstractSection(CreateResources.java:228)
        at widoco.CreateResources.generateDocumentation(CreateResources.java:90)
        at widoco.gui.GuiController.<init>(GuiController.java:275)
        at widoco.gui.GuiController.main(GuiController.java:501)

To Reproduce

Steps to reproduce the behavior:

1. Run WIDOCO through the command line on a machine where the GUI cannot be executed
2. Use an behaviour that requires the GUI (for instance, running on a previously generated folder without the -rewriteAll option)
3. See the error shown above

Expected behavior

I see two possibilities:

• An clear statement is printed explaining that (and perhaps why) the specific operation requires the GUI
• Make all operations also available via the command line