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/udprelay.py

@@ -69,7 +69,7 @@ import struct
 import errno
 import random
 
-from shadowsocks import encrypt, eventloop, lru_cache, common
+from shadowsocks import encrypt, eventloop, lru_cache, common, utils
 from shadowsocks.common import parse_header, pack_addr
 
 
@@ -208,7 +208,7 @@ class UDPRelay(object):
             if err in (errno.EINPROGRESS, errno.EAGAIN):
                 pass
             else:
-                logging.error(e)
+                utils.print_exception(e)
 
     def _handle_client(self, sock):
         data, r_addr = sock.recvfrom(BUF_SIZE)