Bug 1317592 - don't try syncing other engines after one fails with a shutdown error. r=rnewman

MozReview-Commit-ID: LZ72JbOtpm4

--HG--
extra : rebase_source : 00493ea6d24470e5b45a2d42e8b64468427f89d3

services/sync/modules/stages/enginesync.js

@@ -329,6 +329,13 @@ EngineSynchronizer.prototype = {
         // appropriate value.
         return false;
       }
+      // Note that policies.js has already logged info about the exception...
+      if (Async.isShutdownException(e)) {
+        // Failure due to a shutdown exception should prevent other engines
+        // trying to start and immediately failing.
+        this._log.info(`${engine.name} was interrupted by shutdown; no other engines will sync`);
+        return false;
+      }
     }
 
     return true;