- If an error occurred while moving the database file, the data
directory could end up pointing to the new, empty directory after a
restart
- The error message for a full failure was missing the second half that
actually explained that you were supposed to move the data directory
- The check for different-drive migrations didn't work if the new
directory didn't exist (at least on macOS), swallowed some errors, and
interfered with manual migrations from the prefs
- The manual migration button would say that the new directory wasn't
empty even if it just contained .DS_Store
- Don't show "Database migration in progress" after not restarting after
a migration failure
Additionally, after a full failure the migration is now attempted on
every restart and displays a warning each time, since otherwise people
will never move their directories out of the Firefox profile (which is
when it's going to fail the most, due to security software).