Fixes #417 Not fetching each individual rev when _all_docs returns an error

* In CBL_Puller's pullBulkWithAllDocs: method, when _all_docs request fails, all the docs are not treated as leftever docs and will not be fetched individually as the retry process will kick in.

* With this fix, the completed change count issue describing in #417 will be resolved as well.
Merge pull request #422 from couchbase/feature/issue_417

Fixes #417 Not fetch each individual rev when _all_docs returns an error