gRPC Python required RPCs terminating with non-OK status code to still
return a valid response value after calling set_code, even though the
response value was not supposed to be communicated to the client, and
returning None is considered a programming error.
This commit introduces an alternative mechanism to terminate RPCs by
calling the `abort` method on `ServicerContext` passed to the handler,
which raises an exception and signals to the gRPC runtime to abort the
RPC with the specified status code and details.
I made this mistake in 2010985ab2 but
only with yesterday's release of six 1.11.0 has it started failing
("TypeError: metaclass conflict: the metaclassof a derived class must
be a (non-strict) subclass of the metaclasses of all its bases").
ncteisen