Fix running check book with no book opened gives an error message
Edit book: Handle epub files which contain opf files with no <metadata> tags
Fix #1263177 [Typo ("a the") in user-visible message](https://bugs.launchpad.net/calibre/+bug/1263177)
Edit book: Fix crash that happens sometimes when deleting multiple files. Fixes #1263179 [Editor quits while deleting files with "delte" key](https://bugs.launchpad.net/calibre/+bug/1263179)
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Edit book: Fix a typo causing the preferences for which windows occupy which corners to not work