Rename httpError parameter to payload

Change httpError signature from (message, status, body) to (message, status, payload) and set error.payload = payload. Note: the conditional still checks 'body !== undefined', which appears to be an inconsistent leftover and may prevent payload from being attached; update the check to 'payload !== undefined' as a follow-up.

Author: cubap

rest.js

@@ -3,11 +3,11 @@ const acceptedJsonContentTypes = new Set([
   "application/ld+json"
 ])
 
-export function httpError(message, status = 500, body) {
+export function httpError(message, status = 500, payload) {
   const error = new Error(message)
   error.status = status
   if (body !== undefined) {
-    error.payload = body
+    error.payload = payload
   }
   return error
 }