Fewer false mail alerts

• Fix health error messages coming back empty. The sanitizing added for ModSecurity-fronted sites used a regular expression that failed to compile, blanking every error string it touched and logging a PHP warning each time. Paths and error signatures are scrubbed properly now, and URLs survive intact.
• Require three consecutive failures before mail is marked unhealthy, and report consecutive_failures on the email signal. A single bounced address no longer reads as a broken mail transport. Invalid and empty recipient addresses never count, since those are bad form submissions rather than delivery problems.
• Also mark mail unhealthy when two or more failures go an hour with no successful send in between. The consecutive count measures send attempts, not time, so a site sending a couple of mails a day would otherwise stay silent through a total outage.
• Report disk and SSL warnings as "recommended" on the Site Health screen rather than "critical". Both degrade gradually, so neither is the emergency that red implies.
• Widen the scheduled-post grace window from 15 to 30 minutes, so a post published slightly late by a busy cron run is not counted as missed.