update the background color of the progress hud

slightly offwhite, so that if we want to use it without the gradient
mask, it won't totally disappear into a white background.
re-enable sync cancel

don't show SVProgressHUD with a mask during sync. this allows the stop
sync button to still be pressed.
try to avoid crashing in the sync queue

in the event that we can't get a current object out of self.queue, we
should just stop the sync rather than crashing. self.queue may be
getting over-drained due to multiple callbacks from restkit's object
loader in some conditions.