Add optional _raise parameter

Defaults to True but if set to False, when giving up log a warning
instead of raising an exception.

Signed-off-by: Zack Cerza <zack.cerza@inktank.com>
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>
Make try_push_job_info() retry using safe_while

I've noticed sometimes try_push_job_info() fails because of server load
issues. It should try more than once (and now does).

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