Fix issue with DB location changer

I discovered a problem with the database location changer function in my testing, where it would crash if the file didn’t exist.  This is now fixed.
Add pause in “Show all books”

Pause and wait for the user to continue when showing all books.
Change mid-process exit to return

If the user wanted to exit in the middle of, say, adding a book or searching for existing books, it was done by throwing and catching an exception.  I realized that’s a pretty weird way of doing it, so I switched to returning values instead.  Makes more sense.

Also it’s not stated in the UX, but if either the search field or search terms are blank (in searching for local books), it will gently return you to the main screen.
Merge branch 'release/1.0'