Make writeToFile return a Promise in JsonFileManager (#2246)

* Make writeToFile return a Promise in JsonFileManager

* Add return after promise reject

src/common/JsonFileManager.ts

@@ -16,13 +16,18 @@ export default class JsonFileManager<T> {
         }
     }
 
-    writeToFile(): void {
+    writeToFile(): Promise<void> {
+        return new Promise((resolve, reject) => {
             fs.writeFile(this.jsonFile, JSON.stringify(this.json, undefined, 2), (err) => {
                 if (err) {
                     // No real point in bringing electron-log into this otherwise electron-free file
                     // eslint-disable-next-line no-console
                     console.error(err);
+                    reject(err);
+                    return;
                 }
+                resolve();
+            });
         });
     }