Ensure that the test run fails if an uncaught exception occurs

Due to an issue with the interaction between mocha and karma-mocha [1],
uncaught exceptions or promise rejections that occur in between tests
can cause the test run to stop immediately yet incorrectly report that
all tests executed succesfully and hence CI runs / Jenkins builds
incorrectly report their status.

This commit works around the issue until the problem is resolved in
mocha / karma-mocha by adding an `afterEach` hook which will re-throw
any uncaught exceptions and ensure that mocha notices them and reports
the failure correctly to Karma.

[1] See comments on https://github.com/hypothesis/client/issues/2249

src/sidebar/test/bootstrap.js

@@ -23,3 +23,21 @@ registerIcons({
   ...sidebarIcons,
   ...annotatorIcons,
 });
+
+// Ensure that uncaught exceptions between tests result in the tests failing.
+// This works around an issue with mocha / karma-mocha, see
+// https://github.com/hypothesis/client/issues/2249.
+let pendingError = null;
+window.addEventListener('error', event => {
+  pendingError = event.error;
+});
+window.addEventListener('unhandledrejection', event => {
+  pendingError = event.reason;
+});
+
+afterEach(() => {
+  if (pendingError) {
+    console.error('An uncaught exception was thrown between tests');
+    throw pendingError;
+  }
+});