Improved awaitCompletion interface

refs https://github.com/TryGhost/Toolbox/issues/358

- The method is a bit of a dangerous to use in cases when the job takes a long time to execute.
- Returning a boolean value did not make sense and provided no helpful information. Having a job model (or not having one) gives the context in which the "completion" happened.

ghost/job-manager/lib/job-manager.js

@@ -246,9 +246,10 @@ class JobManager {
     }
 
     /**
-     *
+     * Awaits completion of the offloaded job.
+     * CAUTION: it might take a long time to resolve!
      * @param {String} name one-off job name
-     * @returns
+     * @returns resolves with a Job model at current state
      */
     async awaitCompletion(name) {
         const persistedJob = await this._jobsRepository.read({
@@ -262,7 +263,7 @@ class JobManager {
             return this.awaitCompletion(name);
         }
 
-        return !!persistedJob;
+        return persistedJob;
     }
 
     /**