Propagate errors when registering "relatime" property callback.

Various errors can occur when registering property callbacks.  As the
author's comments indicate, the code is very paranoid about preserving
the first-seen error when registering callbacks.  This patch causes an
error seen while registering the "relatime" callback to not clobber a
previously-seen error.

Reported-by: Jorgen Lundman <lundman@lundman.net>