better detect and report error in certain case of sync.
better detect and report error in certain case of sync.

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