Also raise DuplicatedChannel in tester_client
Also raise DuplicatedChannel in tester_client

The corresponding function in `raiden.network.rpc.client` raises a
`DuplicatedChannel` when the channel creation fails. Now `tester_client`
follows this scheme, which allows to handle the exception properly in
`raiden.connection_manager::retry_connect`.

hackaugusto/raiden,hackaugusto/raiden,tomashaber/raiden,tomashaber/raiden,tomashaber/raiden,tomashaber/raiden,tomashaber/raiden