MINOR: config: report when "monitor fail" rules are misplaced

MINOR: config: report when "monitor fail" rules are misplaced

"monitor-uri" may rely on "monitor fail" rules, which are processed
very early, immediately after the HTTP request is parsed and before
any http rulesets. It's not reported by the config parser when this
ruleset is misplaces, causing some configurations not to work like
users would expect. Let's just add the warning for a misplaced rule.