Bug 26699 Create Jar fails with no warning if jar read-only
Once *.jardesc has been created, if I invoke "Create JAR" from the context menu and the jar file already exists and is read-only, the progress dialog flashes and nothing more happens. Please add check for this condition and at least an error message should be displayed. Even better, ask something like "Jar file read-only, overwrite?" Y/N and overwrite the file on yes (similar to what the option "warn if jar file exists" does).
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/jarpackager/JarFileExportOperation.java
private Set fExportedClassContainers;
	private MessageMultiStatus fStatus;
	
	/**
	 * Creates an instance of this class.
	 *
	 * @param	jarPackage	the JAR package specification
	 * @param	parent	the parent for the dialog,
	 * 			or <code>null</code> if no dialog should be presented
	 */
	public JarFileExportOperation(JarPackageData jarPackage, Shell parent) {
		this(new JarPackageData[] {jarPackage}, parent);
	}
	/**
	 * Creates an instance of this class.
	 *
	 * @param	jarPackages		an array with JAR package data objects
	 * @param	parent			the parent for the dialog,
	 * 			or <code>null</code> if no dialog should be presented
	 */
	public JarFileExportOperation(JarPackageData[] jarPackages, Shell parent) {
		this(parent);
		fJarPackages= jarPackages;
	}
	private JarFileExportOperation(Shell parent) {
		fParentShell= parent;
		fStatus= new MessageMultiStatus(JavaPlugin.getPluginId(), IStatus.OK, "", null); 
	}
	protected void addToStatus(CoreException ex) {
		IStatus status= ex.getStatus();