Avoid leaking realms when an error occurs within an event handler

lib/notification-worker.js

@@ -43,7 +43,14 @@ process.on('message', (m) => {
             if (impl.onchange) {
                 const change = Realm.Sync._deserializeChangeSet(m.change);
                 if (!change.isEmpty) {
-                    impl.onchange(change);
+                    try {
+                        impl.onchange(change);
+                    }
+                    catch (e) {
+                        change.close();
+                        process.send({change: m.change});
+                        throw e;
+                    }
                 }
                 change.close();
             }

lib/notifier.js

@@ -65,11 +65,24 @@ class FunctionListener {
             changes.release();
             return;
         }
-        const promise = Promise.resolve(this.fn(changes));
+
+        let promise;
+        try {
+            promise = Promise.resolve(this.fn(changes));
+        }
+        catch (e) {
+            changes.release();
+            throw e;
+        }
+
         this.pending.push(promise);
-        promise.then(() => {
+        const release = () => {
             changes.release();
             this.pending.splice(this.pending.indexOf(promise), 1);
+        };
+        promise.then(release).catch(e => {
+            release();
+            throw e;
         });
     }
 };