Stale-if-error cache-control directive now supported for all responses

Vercel CDN now supports the stale-if-error Cache-Control directive for more resilient caching during origin failures.

• •The stale-if-error directive specifies how long (in seconds) a stale cached response can be served when an origin request fails
• •When the origin returns an error, the CDN may serve a previously cached response instead of returning the error to the client
• •Covered error types include 500 Internal Server Errors, network failures, and DNS errors
• •This allows applications to remain available and respond gracefully during temporary upstream service outages