Show a clearer error if `window.open` call fails

If opening the login popup window using `window.open` fails or succeeds
but fails to return a `Window` reference to the caller then show a more
helpful error message.

There are various rare scenarios (browser bugs, browser extensions)
which might cause this to happen. There is not a whole lot we can do to
recover, but at least we can show a better error message.

src/sidebar/util/oauth-client.js

@@ -255,10 +255,16 @@ export default class OAuthClient {
       })
       .replace(/&/g, ',');
 
-    return /** @type {Window} */ ($window.open(
+    const authWindow = $window.open(
       'about:blank',
       'Log in to Hypothesis',
       authWindowSettings
-    ));
+    );
+
+    if (!authWindow) {
+      throw new Error('Failed to open login window');
+    }
+
+    return authWindow;
   }
 }

src/sidebar/util/test/oauth-client-test.js

@@ -183,7 +183,7 @@ describe('sidebar/util/oauth-client', () => {
     });
   });
 
-  describe('.openAuthPopupWindow', () => {
+  describe('#openAuthPopupWindow', () => {
     it('opens a popup window', () => {
       const fakeWindow = new FakeWindow();
       const popupWindow = OAuthClient.openAuthPopupWindow(fakeWindow);
@@ -195,6 +195,15 @@ describe('sidebar/util/oauth-client', () => {
         'height=430,left=274.5,top=169,width=475'
       );
     });
+
+    it('throws error if popup cannot be opened', () => {
+      const fakeWindow = new FakeWindow();
+      fakeWindow.open = sinon.stub().returns(null);
+
+      assert.throws(() => {
+        OAuthClient.openAuthPopupWindow(fakeWindow);
+      }, 'Failed to open login window');
+    });
   });
 
   describe('#authorize', () => {