Also raise exception on child process, to get a traceback for debugging

I initially thought I could pass the traceback with the exception, but
it seems transmitting exceptions around with their tracebacks intact is
not so easy. There's a library which does this:

https://github.com/ionelmc/python-tblib

But this doesn't seem important enough to introduce a dependency..

subprocess_adapter.py

@@ -70,7 +70,7 @@ def subprocess_operator(cls: Operator, polling_interval=.01) -> Operator:
         try:
             if self.pipe[0].poll():
                 resp = self.pipe[0].recv()
-                assert(isinstance(resp, SubprocessMessage))
+                # assert(isinstance(resp, SubprocessMessage))
             else:
                 resp = None
         except EOFError:
@@ -105,6 +105,7 @@ def subprocess_function(func, *args, pipe, **kwargs):
         except Exception as err:
             log.debug("Caught exception from subprocess: %s", err)
             pipe[1].send(SubprocessMessage(exception=err))
+            raise
         finally:
             pipe[1].close()