implement utils.print_exception()

Previously we used logging.error(e) and traceback.print_exc()
to output error stack trace. The problem is, we want to
output the stack trace only when verbose > 0. The if statement
scattered around the code. So we replaced them with the new
utils.print_exception() call.

shadowsocks/asyncdns.py

@@ -25,7 +25,7 @@ import struct
 import re
 import logging
 
-from shadowsocks import common, lru_cache, eventloop
+from shadowsocks import common, lru_cache, eventloop, utils
 
 
 CACHE_SWEEP_INTERVAL = 30
@@ -221,9 +221,7 @@ def parse_response(data):
                 response.answers.append((an[1], an[2], an[3]))
             return response
     except Exception as e:
-        import traceback
-        traceback.print_exc()
-        logging.error(e)
+        utils.print_exception(e)
         return None