Change JSON result of "job status" when job isn't found.

Instead of just exiting with an error code, return a valid JSON
structure when a user calls "aurora job status", and no job matching
the jobspec is running.

Also, remove a bunch of "pants" labels from build files, which generated
warnings while building the tests.

Testing Done:
Ran all unit tests; added new test for the empty job list json case.

Bugs closed: aurora-803

Reviewed at https://reviews.apache.org/r/26417/