bup server: better error when no servers; also stagger the storage servers update
bup server: better error when no servers; also stagger the storage servers update

tscholl2/smc,DrXyzzy/smc,tscholl2/smc,sagemathinc/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc