Workers reported status via an `osbuild.Result`, which only includes
osbuild output. Make it report OSBuildJobResult instead, which was meant
to be used for this purpose and is already used as the result type in
the jobqueue.
While at it, add any errors produced by targets into this struct, as
well as an overall success flag.
Note that this breaks older workers returning the result of an osbuild
job to a new composer. I think this is fine in this case, for two
reasons:
1. We don't support running different versions of the worker and
composer in the weldr API, and remote workers aren't widely used yet.
2. Both osbuild.Result and worker.OSBuildJobResult have a top-level
`Success` boolean. Thus, logs are lost in such cases, but the overall
status of the compose is not.
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