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
if (fJarPackages.length > 1)
					message= JarPackagerMessages.getString("JarFileExportOperation.creationOfSomeJARsFailed"); 
				else
					message= JarPackagerMessages.getString("JarFileExportOperation.jarCreationFailed"); 
				break;
			default:
				
				message= ""; 
				break;
		}
		fStatus.setMessage(message);
		return fStatus;
	}
	/**
	 * Answer a boolean indicating whether the passed child is a descendant
	 * of one or more members of the passed resources collection
	 *
	 * @param	resources	a List contain potential parents
	 * @param	child		the resource to test
	 * @return	a <code>boolean</code> indicating if the child is a descendant
	 */
	protected boolean isDescendant(List resources, IResource child) {
		if (child.getType() == IResource.PROJECT)
			return false;
		IResource parent= child.getParent();
		if (resources.contains(parent))
			return true;
		return isDescendant(resources, parent);
	}
	protected boolean canBeExported(boolean hasErrors, boolean hasWarnings) throws CoreException {