GSettingsMissingError doesn't have a message attribute, at least under
Python 3.6. This caused a crash in distributions like Debian where
commit 53a8edb7 "desktop: Don't install panel" was reverted to support
Nautilus 3.26, when used in conjunction with a prerelease build of
Nautilus 3.27.

Use the logging module's support for '%' interpolation while I'm
touching those lines anyway, as recommended by pylint warning W1201.