Restore certificate checking for API syncing errors

Closes #864

This adds a 'channel' property to Zotero.HTTP.UnexpectedStatusException,
because the 'channel' property of the XHR can be garbage-collected
before handling, and the channel's 'securityInfo' property is necessary
to detect certificate errors.

test/tests/syncAPIClientTest.js

@@ -0,0 +1,71 @@
+"use strict";
+
+describe("Zotero.Sync.APIClient", function () {
+	Components.utils.import("resource://zotero/config.js");
+	
+	var apiKey = Zotero.Utilities.randomString(24);
+	var baseURL = "http://local.zotero/";
+	var server, client;
+	
+	function setResponse(response) {
+		setHTTPResponse(server, baseURL, response, {});
+	}
+	
+	before(function () {
+		Components.utils.import("resource://zotero/concurrentCaller.js");
+		var caller = new ConcurrentCaller(1);
+		caller.setLogger(msg => Zotero.debug(msg));
+		caller.stopOnError = true;
+		caller.onError = function (e) {
+			Zotero.logError(e);
+			if (e.fatal) {
+				caller.stop();
+				throw e;
+			}
+		};
+		
+		Zotero.HTTP.mock = sinon.FakeXMLHttpRequest;
+		
+		client = new Zotero.Sync.APIClient({
+			baseURL,
+			apiVersion: ZOTERO_CONFIG.API_VERSION,
+			apiKey,
+			caller
+		})
+	})
+	
+	beforeEach(function () {
+		server = sinon.fakeServer.create();
+		server.autoRespond = true;
+	})
+	
+	after(function () {
+		Zotero.HTTP.mock = null;
+	})
+	
+	describe("#_checkConnection()", function () {
+		it("should catch an error with an empty response", function* () {
+			setResponse({
+				method: "GET",
+				url: "error",
+				status: 500,
+				text: ""
+			})
+			var e = yield getPromiseError(client.makeRequest("GET", baseURL + "error"));
+			assert.ok(e);
+			assert.isTrue(e.message.startsWith(Zotero.getString('sync.error.emptyResponseServer')));
+		})
+		
+		it("should catch an interrupted connection", function* () {
+			setResponse({
+				method: "GET",
+				url: "empty",
+				status: 0,
+				text: ""
+			})
+			var e = yield getPromiseError(client.makeRequest("GET", baseURL + "empty"));
+			assert.ok(e);
+			assert.equal(e.message, Zotero.getString('sync.error.checkConnection'));
+		})
+	})
+})