State whether uncaught exception was an exception or a promise

Per PR feedback, be explicit about the kind of exception that occurred.

src/sidebar/test/bootstrap.js

@@ -28,16 +28,20 @@ registerIcons({
 // This works around an issue with mocha / karma-mocha, see
 // https://github.com/hypothesis/client/issues/2249.
 let pendingError = null;
+let pendingErrorNotice = null;
+
 window.addEventListener('error', event => {
   pendingError = event.error;
+  pendingErrorNotice = 'An uncaught exception was thrown between tests';
 });
 window.addEventListener('unhandledrejection', event => {
   pendingError = event.reason;
+  pendingErrorNotice = 'An uncaught promise rejection occurred between tests';
 });
 
 afterEach(() => {
   if (pendingError) {
-    console.error('An uncaught exception was thrown between tests');
+    console.error(pendingErrorNotice);
     throw pendingError;
   }
 });