HTTP.request() improvements

- Move 5xx retries and connection checking out of the sync API client
  and into HTTP.request() so that they apply to all requests. 429 handling
  remains in the API client, since not all callers necessarily want to
  handle that the same way. Callers can still handle 5xx themselves by
  including the relevant 5xx status codes in `successCodes` or by passing
  `errorDelayMax: 0`.
- Add `cancellerReceiver` option, which is a callback that receives a
  function that will cancel the request, whether it's an active request
  or an automatic delay before a 5xx retry.

This also updates Sinon to 7.3.2.

test/content/support.js

@@ -499,8 +499,14 @@ var modifyDataObject = function (obj, params = {}, saveOptions) {
 /**
  * Return a promise for the error thrown by a promise, or false if none
  */
-function getPromiseError(promise) {
-	return promise.thenReturn(false).catch(e => e);
+async function getPromiseError(promise) {
+	try {
+		await promise;
+	}
+	catch (e) {
+		return e;
+	}
+	return false;
 }
 
 /**