Re-raise exception from deliver_task in MQTTClient.deliver_message().
Re-raise exception from deliver_task in MQTTClient.deliver_message().

deliver_task will raise a ClientException to signal a connection loss.
Passing it on to the caller instead of silently ignoring it lets the
caller handle the connection loss. Exiting the function early also
avoids an IndexError when trying to pop from client_tasks.

Fixes #106

beerfactory/hbmqtt