buildbot.service: better handle error

fix for 2960

In case of 404, the buildbot.service was storing the 404 error instead of retrying next time

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Another attempt to fix the element does not exist (yet) problem.

Upon 404, we still define the object, with everything undefined, and register to events

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
fix the watch of nextbuild existence

in case the build was already loaded, callback was never called with o == undefined

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge pull request #1306 from tardyp/t2960

buildbot.service: better handle error

Fixes ticket:2960