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 false;
		}
		
		if (fParentShell == null)
			
			return true;
		IFile[] unsavedFiles= getUnsavedFiles();
		if (unsavedFiles.length > 0)
			return saveModifiedResourcesIfUserConfirms(unsavedFiles);
		return true;
	}
	/**
	 * Returns the files which are not saved and which are
	 * part of the files being exported.
	 * 
	 * @return an array of unsaved files
	 */
	private IFile[] getUnsavedFiles() {
		IEditorPart[] dirtyEditors= getDirtyEditors(fParentShell);
		Set unsavedFiles= new HashSet(dirtyEditors.length);
		if (dirtyEditors.length > 0) {
			List selection= JarPackagerUtil.asResources(fJarPackage.getElements());
			for (int i= 0; i < dirtyEditors.length; i++) {
				if (dirtyEditors[i].getEditorInput() instanceof IFileEditorInput) {
					IFile dirtyFile= ((IFileEditorInput)dirtyEditors[i].getEditorInput()).getFile();
					if (JarPackagerUtil.contains(selection, dirtyFile)) {
						unsavedFiles.add(dirtyFile);
					}
				}
			}