The recent changes removed the {Assembler,Stage}Failed exceptions,
which includes them being thrown from Stage.run and Assembler.run.
Instead result dictionaries are returned even on errors. But the
object store, used as a context manager, relies on exceptions to
detect the error case and thus needs them to cleanup the temporary
objects. Without those exceptions the temporary objects end up in
the store even when the sage or assembler failed.
Restore the old behavior by throwing a generic BuildError exception
from the Stage and Assembler, which will be caught directly in the
pipeline and converted to a result dict.
ede3f6baeb