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
* @return true if user pressed OK and save was successful.
	 */
	private boolean saveModifiedResourcesIfUserConfirms(IFile[] dirtyFiles) {
		if (confirmSaveModifiedResources(dirtyFiles))
			return saveModifiedResources(dirtyFiles);
		
		for (int i= 0; i < dirtyFiles.length; i++)
			addError(JarPackagerMessages.getFormattedString("JarFileExportOperation.fileUnsaved", dirtyFiles[i].getFullPath()), null); 
		return false;
	}
	/**
	 * Save all of the editors in the workbench.  
	 * 
	 * @return true if successful.
	 */
	private boolean saveModifiedResources(final IFile[] dirtyFiles) {
		
		Display display= fParentShell.getDisplay();
		if (display == null || display.isDisposed())
			return false;
		
		final boolean[] retVal= new boolean[1];
		Runnable runnable= new Runnable() {
			public void run() {
				try {
					new ProgressMonitorDialog(fParentShell).run(false, false, createSaveModifiedResourcesRunnable(dirtyFiles));
					retVal[0]= true;
				} catch (InvocationTargetException ex) {
					addError(JarPackagerMessages.getString("JarFileExportOperation.errorSavingModifiedResources"), ex); 
					JavaPlugin.log(ex);