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
* Answers whether the given resource is a Java file.
	 * The resource must be a file whose file name ends with ".java".
	 * 
	 * @return a <code>true<code> if the given resource is a Java file
	 */
	boolean isJavaFile(IResource file) {
		return file != null
			&& file.getType() == IFile.FILE
			&& file.getFileExtension() != null
			&& file.getFileExtension().equalsIgnoreCase("java"); 
	}
	/**
	 * Answers whether the given resource is a class file.
	 * The resource must be a file whose file name ends with ".class".
	 * 
	 * @return a <code>true<code> if the given resource is a class file
	 */
	boolean isClassFile(IResource file) {
		return file != null
			&& file.getType() == IFile.FILE
			&& file.getFileExtension() != null
			&& file.getFileExtension().equalsIgnoreCase("class"); 
	}
	/*
	 * Builds and returns a map that has the class files
	 * for each java file in a given directory
	 */
	private Map buildJavaToClassMap(IContainer container) throws CoreException {
		if (!isCompilerGeneratingSourceFileAttribute())
			return null;