NewsManager.getUnreadItems: handle EROFS (490732)

When getUnreadItems tries to lock the news.unread file, it's safe to
ignore EROFS. This is handled with a ReadOnlyFileSystem exception
raised from the portage.locks.lockfile function.

X-Gentoo-Bug: 490732
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=490732