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/server.py

@@ -80,10 +80,7 @@ def main():
             daemon.set_user(config.get('user', None))
             loop.run()
         except Exception as e:
-            logging.error(e)
-            if config['verbose']:
-                import traceback
-                traceback.print_exc()
+            utils.print_exception(e)
             sys.exit(1)
 
     if int(config['workers']) > 1: