BUG/MEDIUM: proxy: protect "show errors" against backend deletion

BUG/MEDIUM: proxy: protect "show errors" against backend deletion

Command "show errors" is not safe as it loops over the list of proxies.
A crash may occur if the task yields on a proxy which is deleted just
before the dump is restarted.

As with previous fixes dealing with similar cases, this is fixed via the
watcher mechanism. A new <px_watcher> member is defined in
<show_errors_ctx> and used to loop over the proxies list.

This must be backported up to 3.4.