The exception that was thrown by {stage.run, assembler.run} was a
necessary ingredient that in combination with the context manager
around `Objectstore.new` made sure that tree the object was only
auto-committed to the store when there was no error during the
executing of any of the `.run` methods.
Now that the auto-commit feature got removed and committing of any
object to the store is explicitly done via `objectstore.commit`,
the whole exception throwing and handling can be removed. Status
reporting was already done in `BuildResult.success` and the new
code will use that to exit the function early on stage/asm errors.
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