.ix does not warn when selecting a none-existing column
```
df3 = DataFrame({"a":[1,2,3,4], "b":[1,2,3,4]})
df3.ix[:,["b","c"]]
Out[1]: 
   b   c
0  1 NaN
1  2 NaN
2  3 NaN
3  4 NaN
```

I would have (had...) expected that the `df3.ix[:,["b","c"]]` will throw an error :-(
