Rescue errors when fetching nested job

When the waylon config referenced a nested view that did not exist the
Jenkins API client would throw an exception that would propagate up the
stack and return to the client. This commit fixes the code to return a
404 and appropriate message when a server is configured to return a
non-existent job.
Merge pull request #53 from adrienthebo/bug/master/jenkins-server-missing-nested-views

Rescue errors when fetching nested job