Add optional failure handler for XHR effect

* This extends the options for handling `xhr.send()` failures, in
  the provided effect. It allows users to handle the exception
  thrown on calls to `send()` explicitly, making it more capable to
  handle or recover from network failure, or loss of service, if
  the users whishes to.

src/batteries.js

@@ -78,6 +78,7 @@ effector(
     data,
     onSuccess,
     onError,
+    onFailure,
   }) => {
     const xhr = new XMLHttpRequest();
     xhr.open(method, url);
@@ -108,6 +109,14 @@ effector(
           }
       }
     };
-    xhr.send(data);
+    try {
+      xhr.send(data);
+    } catch (e) {
+      if (onFailure) {
+        trigger(...onFailure, { error: e });
+      } else {
+        throw e;
+      }
+    }
   },
 );

src/batteries.test.js

@@ -7,16 +7,23 @@ import { connect } from "./connector";
 
 jest.mock("./event");
 
+let THROW_ERROR_ON_SEND = false;
+
 let xhrs = [];
 beforeEach(() => {
+  THROW_ERROR_ON_SEND = false;
   xhrs = [];
   trigger.mockClear();
 });
 
 window.XMLHttpRequest = function() {
   let xhr = {
     open: jest.fn(),
-    send: jest.fn(),
+    send: THROW_ERROR_ON_SEND
+      ? () => {
+          throw THROW_ERROR_ON_SEND;
+        }
+      : jest.fn(),
     setRequestHeader: jest.fn(),
   };
   // remember created xhr requests
@@ -125,6 +132,15 @@ describe("xhr effect", () => {
     expect(trigger).toHaveBeenCalledTimes(1);
     expect(trigger).toHaveBeenCalledWith("error", "foo", response);
   });
+  it("triggers an event on failure", () => {
+    THROW_ERROR_ON_SEND = new Error("no soup for you!");
+    effect("xhr", {
+      onFailure: ["failure", "foo"],
+    });
+    expect(trigger).toHaveBeenCalledWith("failure", "foo", {
+      error: THROW_ERROR_ON_SEND,
+    });
+  });
   it("ignores success if onSuccess is unset", () => {
     effect("xhr", {});