Propagate exit code from block process to top-level process

[vsergeev]

When a child block process exits due to a runtime error, the error is not propagated to the top-level process. The exit information is available from waitpid() in CompositeBlock:_reap(), and could be returned to CompositeBlock:wait(), and then returned to the user, so the user can decide how to handle the erroneous exit.