Sync engine cleanup

- Use custom exception for user-initiated sync cancellations, which can bubble
  up to the sync runner -- this should help with a sync stop button (#915)
- Separate out deletions-downloading code
- Refactor delay generator handling on library version mismatch
- Clearer variable names

components/zotero-service.js

@@ -104,6 +104,7 @@ const xpcomFilesLocal = [
 	'sync',
 	'sync/syncAPIClient',
 	'sync/syncEngine',
+	'sync/syncExceptions',
 	'sync/syncEventListeners',
 	'sync/syncFullTextEngine',
 	'sync/syncLocal',