Display Rate Limiting error message when response is 429

[nicomollet]

Is your feature request related to a problem? Please describe. When the user/domain is rate limited by SaaS, the performance monitoring job returns 429. The issue is the plugin reports a "Firewall" issue in that case, and it is misleading customer and support.

{
    "code": 429,
    "error": "Rate limit",
    "message": "Host is currently rate limited. Please retry later."
}

Describe the solution you'd like We want to have a better wording in this scenario. TODO @piotrbak

Describe alternatives you've considered N/A

Additional context N/A

[piotrbak]

https://github.com/wp-media/wp-rocket/pull/7770#issuecomment-3370693764