Add a test cases for how MultiplexingCache handles errors

Hello @dpup, 

Please review the following commits I made in branch 'xiao-add-test-cases'.

d0c509f9d5335364ea942739910e21b4b0f15278 (2014-05-28 12:12:29 -0700)

```
Add a test cases for how MultiplexingCache handles errors

Currently MultiplexingCache swallows errors, which I don't think it should
do. I will think of a way to handle errors. This commit just add an explicit
test case to reveal the current behavior.
```

R=@dpup