Application error arguments get encoded as bytes

i ran across this weird behavior when using Autobahn-Js as a client for a backend made using Autobahn-Pyton, it seems that when using python 2.7 with msgpack serializer the arguments of an error get encoded as bytes. this causes them to end up in Base64 in a client that doesn't use msgpack .

to me this seems undesirable?

Backend component:

from autobahn.twisted.component import Component, run

def main(session, details):

    def test(val=5):
        return 100/val

    session.register(test, u'test.hello')

# Create wamp conn
server = Component(
    transports=[{
        'url': u'ws://localhost:9000',
        'serializers': ['msgpack']
    }],
    realm=u'test',
)
server.on_join(main)

if __name__ == "__main__":
    run([server])

Client:

from autobahn.twisted.component import Component, run

def main(session, details):

    session.call(u'test.hello',7)
    session.call(u'test.hello',0)
    print('Done')

# Create wamp conn
client = Component(
    transports=[{
        'url': u'ws://localhost:9000'
    }],
    realm=u'test',
)
client.on_join(main)

if __name__ == '__main__':
    run([client])

Running the backend and client gives the below output on the client

2019-01-16T16:03:23+0100 connecting once using transport type "websocket" over endpoint "tcp"
2019-01-16T16:03:23+0100 Starting factory <autobahn.twisted.websocket.WampWebSocketClientFactory object at 0x7fe1f4a3e550>
2019-01-16T16:03:23+0100 Done
2019-01-16T16:03:23+0100 Unhandled error in Deferred:
2019-01-16T16:03:23+0100 Traceback (most recent call last):
Failure: autobahn.wamp.exception.ApplicationError: ApplicationError(error=<wamp.error.runtime_error>, args=[u'aW50ZWdlciBkaXZpc2lvbiBvciBtb2R1bG8gYnkgemVybw=='], kwargs={}, enc_algo=None)

as can be seen a 'integer division or modulo by zero' causes this odd behavior to appear

Start by reproducing the Python 2.7 backend and Autobahn-JS client combination shown in the issue, using the msgpack serializer on the backend and triggering the division-by-zero error. Trace how the application error arguments are serialized and decoded; done means a client without msgpack receives the error text as readable arguments rather than Base64 bytes, with a regression test for this case.