client: Strip remote dbus error when setting metadata fails

Without this, if the function fails, the error presented to the user looks
like:

GDBus.Error:org.gtk.GDBus.UnmappedGError.Quark._g_2dio_2derror_2dquark.Code1:
Can't find metadata file /home/ross/.local/share/gvfs-metadata/home

https://bugzilla.gnome.org/show_bug.cgi?id=598561