control plane: add request id to all error pages (#2149)

* controlplane: add request id to all error pages

- use a single http error handler for both envoy and go control plane
- add http lib style status text for our custom statuses.

Signed-off-by: Bobby DeSimone <bobbydesimone@gmail.com>

internal/httputil/httputil.go

@@ -4,3 +4,13 @@ package httputil
 // client's certificate is invalid. This is the same status code used
 // by nginx for this purpose.
 const StatusInvalidClientCertificate = 495
+
+var statusText = map[int]string{
+	StatusInvalidClientCertificate: "a valid client certificate is required to access this page",
+}
+
+// StatusText returns a text for the HTTP status code. It returns the empty
+// string if the code is unknown.
+func StatusText(code int) string {
+	return statusText[code]
+}