don't run afterSyncCompleted when there was an error

src/matrix/Sync.js

@@ -100,6 +100,7 @@ export class Sync {
                     this._status.set(SyncStatus.Stopped);
                 }
             }
+            if (!this._error) {
                 try {
                     await this._session.afterSyncCompleted();
                 } catch (err) {
@@ -108,6 +109,7 @@ export class Sync {
                 }
             }
         }
+    }
 
     async _syncRequest(syncToken, timeout) {
         let {syncFilterId} = this._session;