MINOR: mworker: Update messages referencing exit-on-failure

Commit 4cfede87a313456fcbce7a185312460b4e1d05b7 removed
`exit-on-failure` in favor of `no-exit-on-failure`, but failed
to update references to the former in user facing messages.

This should be backported to haproxy 1.8.
1 file changed