Improve http error messages; cleanup

Improve http error messages; cleanup

By default QNetworkReply::errorString() often produces messages like
   "Error downloading <url> - server replied: <reason>"
but the "downloading" part invariably confuses people since the
error might very well have been produced by a PUT request.

This commit produces clearer error messages for HTTP errors.

Additionally:
* Remove some unnecessary null checks from slots connected to
  network job signals and document that these signals never send
  null replies.
* There was a bug where AbstractNetworkJob::_timedout wasn't
  set when derived classes overrode slotTimeout. We now ensure
  it's always set by disallowing overrides of slotTimeout.
  Instead it now calls onTimedOut, which allows custom handling.
* Several subclasses declared errorString, isTimedOut. Move
  these to AbstractNetworkJob.
* Unify handling of OC-ErrorString (via the new, general
  Job::errorString)
* Add documentation in various places.