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.