[D7] Option to do not delete the job on exception

Problem/Motivation

Same as #3351024: Option to do not delete the job on exception for Drupal 7.

Problem: when there is a PDOException from the database during your cron job run then the cronjob will be deleted. That is very bad when I expect the cronjob to run every hour and the exception was just temporary.

Steps to reproduce

Throw an exception in the callback of a job. Observe how the job is dropped from the job_schedule after execution.

Proposed resolution

Add a variable to never drop jobs.

Remaining tasks

Patch review.