Fix #1291085 [Add 'tooltip'(title) to Author in book details](https://bugs.launchpad.net/calibre/+bug/1291085)
Library backup: Avoid infinite retries if converting metadata to backup OPF for a book fails. Simply fail to backup the metadata for that book. Fixes #1291142 [Memory Error/ Failed to convert to opf for id:](https://bugs.launchpad.net/calibre/+bug/1291142)