Update safe_while's suggested usage pattern

Update safe_while's suggested usage pattern

I didn't love the way safe_while was encouraged to be used and it didn't
fit right with the new no-raising behavior. Now it's encouraged to be
used like this:

with safe_while() as proceed:
    while proceed():
        do_things()

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>