[caldav] Improve error handling, and only send REPORT if other requests succeeded

- In quick sync mode, don't send a REPORT if previous requests
  failed (i.e. don't download changes if any of the change upload
  operations failed)
- Abort pending requests if any request fails
- Simplify error and result reporting in Request classes
- Remove Request::slotError() as we can detect the error in the slot
  connected to the QNetworkReply::finished() signal
[caldav] Only look for incidence changes in the notebook of the relevant account

Use the notebook created in startSlowSync() -- i.e. the one created
for this account. We don't want to upload calendar events that belong
to other accounts or local calendars.
[caldav] Don't send incidences found in both inserted and removed changes
[caldav] Rename otherRequestFinished() and change emitted error type
Merge pull request #5 from nemomobile/more-fixes

Improve request processing, use correct notebook, and handle notebook change case