The osbuild export is specific to the upload target and different
targets may require using a different export. While osbuild-composer
still does not support multiple exports for osbuild jobs, this prepares
the ground for such support in the future.
The backward compatibility with older implementations of the composer
and workers is kept on the JSON (Un)mashaling level, where the JSON
message is always a super-set of the old and new way of providing the
exports to osbuild job.
Add a new worker client error type `ErrorTargetError` representing that
at least one of job targets failed. The actual target errors are added
to the job detail.
Add a new `OSBuildJobResult.TargetErrors()` method for gathering a slice
of target errors contained within an `OSBuildJobResult` instance. Cover
the method with unit test.
