Jenkins report plugin: fix spurious errors when no completed build for a given job

When some jobs in the reports view do not have any build, currently an error is shown to the user.

For example, I got:

Error while retrieving jobs from Jenkins for url http://qa.nuxeo.org/jenkins/job/addons_FT_nuxeo-birt-5.8/lastCompletedBuild/api/json: http://qa.nuxeo.org/jenkins/job/addons_FT_nuxeo-birt-5.8/lastCompletedBuild/api/json

because there was no last completed build for this job.

And if there are multiple jobs in that situation, only the first one is shown.

This should be presented differently, for instance adding a new status type for that kind of job (but that would mean adding it to the list, e.g. count it in failures)