Clarify job_exists() error messages

To verify a job exist, we query Jenkins API and attempt to load the
resulting JSON.  The error message is very simple:

 Jenkins returned an unexpected job name

This patch add the actual job name received and the expected one.  That
might help diagnosis.

Change-Id: Ib13c3bb8d265612811714d10818364956f84a341
Merge "Clarify job_exists() error messages"