This commit gives more visibility into job failures by making the
information regarding a job runtime error available in an extension
table (`job_errors`) that users can directly query.
This commit also adds an infromational view on top of the table for
convenience.
To prevent the `job_errors` table from growing too large,
a retention job is also set up with a default retention interval
of 1 month. The retention job is registered with a custom check
function that requires that a valid "drop_after" interval be provided
in the config field of the job.
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