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
* @param	exception	the throwable that caused the error, or <code>null</code>
	 */
	protected void addError(String message, Throwable error) {
		fStatus.add(new Status(IStatus.ERROR, JavaPlugin.getPluginId(), JavaStatusConstants.INTERNAL_ERROR, message, error));
	}
	/**
	 * Answers the number of file resources specified by the JAR package.
	 *
	 * @return int
	 */
	protected int countSelectedElements() {
		int count= 0;
		int n= fJarPackage.getElements().length;
		for (int i= 0; i < n; i++) {
			Object element= fJarPackage.getElements()[i];
			IResource resource= null;
			if (element instanceof IJavaElement) {
				IJavaElement je= (IJavaElement)element;
				try {
					resource= je.getUnderlyingResource();
				} catch (JavaModelException ex) {
					addWarning(JarPackagerMessages.getFormattedString("JarFileExportOperation.underlyingResourceNotFound", je.getElementName()), ex); 
					return count;
				}
			}
			else
				resource= (IResource)element;
			if (resource.getType() == IResource.FILE)
				count++;
			else