[macbri]

I know this is an older question, but if you're still interested in a solution:

You could check the exit status of the mount_smbfs command. Something like:

mount_smbfs //bones@storage/Timbackup /Volumes/Timbackup
if [ $? = 0 ];
then
    rsync -vaE --delete ... etc etc.  
    umount /Volumes/Timbackup
fi