Improve log message "context deadline exceeded" with beta_health check

[johakoch]

If the beta_health-checked backend does not respond in time, the logged message is "backend error: context deadline exceeded", which could be more user-friendly.
Example:

server {
  hosts = ["*:8080"]
  endpoint "/" {
    proxy {
      backend = "be"
    }
  }
}

definitions {
  backend "be" {
    origin = "https://blackhole.webpagetest.org"
    beta_health {
    }
  }
}