add missing imports to readme (#43)

Author: ajrice6713

README.md

@@ -24,8 +24,10 @@ pip install bandwidth-sdk
 from bandwidth.bandwidth_client import BandwidthClient
 
 from bandwidth.messaging.models.message_request import MessageRequest
+from bandwidth.messaging.exceptions.messaging_exception import MessagingException
 
 from bandwidth.voice.models.create_call_request import CreateCallRequest
+from bandwidth.voice.exceptions.api_error_exception import ApiErrorException
 from bandwidth.voice.bxml.response import Response
 from bandwidth.voice.bxml.verbs import *