ENSession: do not fail processing linked notebooks when fetching one of the shared notebook record fails

1. This commit is to fix https://github.com/evernote/evernote-cloud-sdk-ios/issues/45
2. Fetching shared notebook record can fail because of various reasons. The owner of the shared notebook could delete the notebook and we'll hit a failure here
3. When we hit a failure when fetching the shared notebook record, log it and continue

Change-Id: Ie3f601ccfa4d753ae765caced5a34a3edced6284