Add new `JobDependencyChainErrors()` method for gathering a stack trace
of job errors from the job's dependencies which caused it to fail.
The `JobDependencyChainErrors()` implementation uses job-type specific
`...Status()` methods intentionally, because job-type specific status
methods check the job's result in a slightly different way and set
the result.JobError to a specific value. Due to this reason, it would
not be practical to introduce a generic `JobStatus()` method and get rid
of the `switch` block, because in reality, the new method would have
to implement an equivalent `switch` block as well.
Add unit test covering the method functionality.
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