Fix README example code.

README.md

@@ -46,6 +46,7 @@ Note that this requires `git` to be installed.
 
 ```py
 import discord
+import asyncio
 
 client = discord.Client()
 
@@ -76,7 +77,11 @@ def main_task():
     yield from client.connect()
 
 loop = asyncio.get_event_loop()
+try:
     loop.run_until_complete(main_task())
+except Exception:
+    loop.run_until_complete(client.close())
+finally:
     loop.close()
 ```